Y=(kx)/z
15=(k10)/4
15=(5k/2)
15=k(5/2)
ties both sides by 2/5 to clear fraction
6=k
y=(6x)/z
x=20
z=6
y=(6*20)/6
y=20
Suppose you can factorize:

Then by expanding the right side, you have

which means in this case, you have to have
Answer: y = 9x - 3
That's y=mx + b, the usual slope intercept form of a line with slope m and y intercept b.