What are these called? i completely forgot. what is the x value such that y =15 - 3x and y = 0?

Your answer is x=5.

Y = 15-3x
Substitute 0 in for y (given). Then,solve the equation for x.

0 = 15 -3x
Subtract 15 to both sides.

-15 = -3x
Divide both sides by -3.

5= x

Problem: y = 15 - 3x;  0 = 15 - 3x

I will try to solve your system of equations:

0 =  - 3x + 15;  y =  - 3x + 15

Solve 0 =  - 3x + 15 for x:

0 + 3x =   - 3x + 15 + 3x  (add 3x to both sides)

3x = 15

3x/3  = 15/3   ( divide both sides by 3)

x  = 5

Substitute 5 for x in y =  - 3x + 15

y =  - 3x + 15

y = ( - 3)(5) + 15

y = 0  (Simplify both sides of the equation)

x = 5  and y = 0

Hope that helps