input-------> +3 -------->x5 -------> output. -HELP-?

If x is the input then it is changed to x+3, and that is then multiplied by 5 giving you an output of 5(x+3).

You are told that the output equals 3 times the input, so it must also equal 3x. The two outputs for the same input must be equal (otherwise it is not a function), so

5(x+3) = 3x

Multiply out the left side and solve

5x + 15 = 3x, subtract 3x from both sides
2x + 15 = 0, so
2x = -15, so
x = -15/2 (i.e. -7 1/2)

output = 3 * input
You have more variables, 2, than equations, 1, so there are infinitely many solutions.