I am going to assume by simplify you mean find the value of x. I am also assuming that the variable x is being multiplied by the denominator not the entire fraction from the way you wrote it.

-15/35x -7 = 0           set up the initial equation

-15/35x = 7               add 7 to both sides

-15 = 245x                multiply both sides by 35x

-15/245 = x               divide both sides by 245

-3/49 = x                   simplify the fraction

Just because I am unsure about how the equation that you posted is set up I will solve it assuming that the equation is (-15/35)x - 7

(-15/35)x - 7 = 0        set up the initial equation

(-3/7)x - 7 = 0           simplify the fraction

(-3/7)x = 7                add 7 to both sides

x = -49/3                  multiply both sides by -(3/7)

Zack Misso