* If -4x = 20: Then x = -5. Substituting this into y = x + 15 gives y = -5 + 15 = 10.
* If -4x is the result of an operation (e.g., -4x = y): Then y = x + 15 and y = -4x. We can solve this system of equations:
Substitute the first equation into the second: x + 15 = -4x
Add 4x to both sides: 5x + 15 = 0
Subtract 15 from both sides: 5x = -15
Divide by 5: x = -3
Substitute x = -3 into y = x + 15: y = -3 + 15 = 12
* If something else is happening with -4x: Please provide the complete question.