Hey ineedhelplz.

The factor is taken into account within the limits of the new integral. Let z*5 = x. 5*dz = dx/5. If the old limits for z were [0,a] then when z = 0, x = 0 and when z = a, x = 5a. This is just an integral substitution that retains the value of the integral, but compensates for the change in variable by changing the limits.