To make the equation easier to look at change it to this:
y = 15 - 3x
To find the x intercept say y = 0.
0 = 15 - 3x
Subtract 15 from each side.
-15 = -3x
Divide each side by -3.
5 = x <–––– x-intercept
The y-intercept of the equation is 15.
Hope this helps!