Question

What is the x value with that y =15 -3x and y =0?

Answer 1

$\bf y=15-3x~\hspace{5em} \stackrel{y=0}{0=15-3x}\implies 3x=15\implies x=\cfrac{15}{3}\implies x = 5$

Answer 2

The answer is 5, since y=0 you need the other side to equal 0 as well. So substituting 5 for x looks like this 0=15-3(5), 0=15-15, 0=0