This is a very very simple question about vectors:
To find the acute angle between a line and a plane, you use the formula
cosx = (scalar product between normal of plane and directional vector of line)/(product of modulus of normal and directional vector)
After that, do 90 degrees minus x to get the answer.
(By the way, I realise that using sinx=.... would spare me the effort of minusing - but is that way recommended?)
In this question of plane 2x-y+4z=9 and directional vector (10,5,-5), after doing all the working, I get cosx= -0.089... and x=95.11 degrees.
But the question wants acute angle and 90-x gives me -5.1 degrees. What should I do? The answer is 5.1 deg but I don't see how 90-x gets me that...
NB: using sinx gives me -5.1 degrees straightaway.