Pretty much you’re going to need to subtract the -9x to get it on the other side, so that you get y= 9x+6
Just pretend it’s a 3d right angle triangle and find the length of the hypotenuse.
d = sqrt(6^2 + 10^2 + 15^2) = 19
Compatible 50 x 120 = 6000
Rounding 50 x 123 =6150
Compatible numbers is closer estimate