To make this easier, you need to change the fractions by multiplication.
-3/4x=15 - First, you multiply 4 to both sides of the equation
(4)-3/4x=15(4)
-12/4x=60 - Simplify
-3x=60 - Divide by -3 on both sides
x=-20 -> (-20,0)
Do the same to find the y-intercept, to get (0,18)