Rachel had to complete w pages of math homework, x pages of reading homework and y more pages of biology homework. How many pages of math and biology homework did she have to complete?
Let's denote the number of pages of math homework by m, the number of pages of reading homework by r, and the number of pages of biology homework by b.

m = w
r = x
b = y

Therefore, the answer (symbolic expression) is: 

m + b