Find a constant between these numbers I am trying to relate a set of body weight measurements on a graph to body fat $\%$ on the same graph.  The closest I can come in terms of the mathematics required, is that it seems to be linear algebra.
I need some help to find a constant for a linear equation that I can use to calculate a set of values for $y$ when given $x.$  
I have been given $3$ examples to work off:
When $x = 85.7, y = 14.3;$ when $x = 84.2, y = 12.8;$ when $x = 83.2, y = 11.8$
What is the constant?
Thank you for any help you can give me.
Regards,
Ryan
 A: When graphing an affine function, i.e. a polynomial of degree 1, you want an equation of the form $y=mx+b$, such that $x$ is its input value, $y$ its corresponding output value, $m$ the constant rate of change of the curve's path (often called its "slope"), and $b$ the y-intercept. (Where the function crosses the y-axis at some point $(0,a)$.
In your above question, we simply want to determine the average rate of change of the function, that is the change in its relative y-coordinates, divided by the change in its corresponding x-coordinates.
This is noted more mathematically as: $m=\frac {y_2-y_1}{x_2-x_1}$
So pick two sets of points!
Let's pick $x = 85.7, \ y= 14.3$ and $x = 84.2, \ y = 12.8$
So we have:
$$m=\frac {y_2-y_1}{x_2-x_1}=\frac {12.8-14.3}{84.2-85.7}=\frac {-1.5}{-1.5}=1$$
So the slope $m=1$
What does this mean however? Well as you shift one unit to the right of the curve, its y value increases by 1. 
Try the same method, but with the third point. What do you notice?  
To determine the equation $y=mx+b$, pick any of your three points for $y$ and $x$, substitute your slope value $m$ in, and solve for $b$.
$$y=mx+b\Rightarrow 14.3=85.7+b\Rightarrow b=-71.4$$
So your final equation is: $y=x-71.4$ 
Try this same method, but using a different combination of points, do you get the same equation?
