Trigonometry basic question I am training Trigonometry just for fun, so I am not in a hurry, but would like to know how to answer this question - not the result, but how to do it. Sorry because I understand this is too basic for you, but I forgot almost everything from elementary school...
Question says: 

A sheet of paper measures $210  \space mm \times 297 \space mm$. Consider a diagonal from
  one corner of the sheet to its opposite corner, and choose a point on
  the diagonal so that its distance to the furthest edge of the sheet is
  equal to the length of that edge, that is, $210 \space mm$. What is the
  distance of the point to the nearest edge of the sheet?

This is the point I reached (did it in Paint - sorry about the quality): 

So as you can see I tried but with no success, and would like to know how to continue. Someone can help me? 
Thank you very much in advance.
 A: Wouldn't "distance to the furthest edge" be from the point to one of the edges (not a corner)? So, we can take the vertical line from your 'x" mark to the bottom edge to have length 210mm. If this is the case, you want to use similar triangles.  

Warning: solution follows:

Draw a horizontal line from the point denoted by the "x" mark on your diagram to the right side of your rectangle and a vertical line from the point to the bottom side. This gives you two similar triangles: the "bottom" one, call it $B$, and the "top one", call it $T$.
Let $a$ be the length of the horizontal  side of $B$  and let $h$ be the height of the vertical side of $T$. Let $c$ be the length of the horizontal side of $T$ (see diagram below).
As remarked above, we set things up so that  the vertical side of $B$ has length $210$ and $h=87$.
A ratio of the non-hypotenuse sides of $B$ is 
${210\over a}$.
The corresponding ratio of the non-hypotenuse sides of $T$ is 
${87\over c}$.
The corresponding ratio of the non-hypotenuse sides of the lower triangle formed by the diagonal and the rectangle is ${297\over210}$. 
Since the three triangles are similar
$$
{210\over a}={87\over c}={297\over 210}.
$$
From this, you can find the values of $a$ and $c$.  Then you can find the shortest distance to an edge.

Not to scale. 

