In order to vertically stretch a function by a factor k, we have to multiply the function expression by k.
So, in order to stretch the function by a factor of 1/5, we simply have to multiply:
![f(x) = -5x \implies g(x) = -5x\cdot(1)/(5)=-x](https://img.qammunity.org/2020/formulas/mathematics/middle-school/pjgjawtj1b9yio3n551dlv4gqxzq92v79n.png)
So, if we shrink the function -5x by a factor of 1/5, we get the function -x.