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

Y=15-3x substitute 0 for y

(0) = 15-3x subtract 15 from both sides

-15=-3x divide by - 3 on both sides, a negative divided by or times by another negative will always equal a positive so x equals positive 5

x=5

X=5

because 15-3 (5) = 15

So, 15-15=0

X=5