How do you solve an equation with the variable in the denominator

Homework Statement

0 = $$\sum n_i / (b-a_i)$$
the summation is goes from i = 1 to k

the question is to rationalize each term so that the denominator is positive, and then solve for b.

The Attempt at a Solution

i tried to expand the summation and get the factor of b and stuff
but it didn't work at all
does anyone know how to solve this summation in terms of b? thanks