Suppose m<n. Prove that the intervals (0,1) and (m,n) are equinumerious by finding a specific bijection between them.

I used the line formula to get $\displaystyle f(x) = \frac{1}{n-m}(x-m)$, where m<n.

I got this question wrong, and im wondering why? is it because it asked for a specific bijection? if so, how would I find one from what is given?