Answer:
-2x
Explanation:
x - 3x is = -2x (i got this from the left side of the equation)
and you need both sides of the equation to be equal to each other for it to be infinite number of solutions. since -2x is the same as x - 3x and both sides of the equations have +15, then you need to find the value that would make the equations the same. and -2x is that