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?
First, we need to find out how many pages of math and biology homework Rachel had to complete. 

We know that: 

Rachel had to complete w pages of math homework, 

x pages of reading homework, 

and y more pages of biology homework. 

That means that she had to complete w + y pages of math and biology homework. 

We also know that: 

w + x + y = 18 

That means that: 

w + y = 18 - x 

Now we have all the information we need to solve the task. 

We need to find out how many pages of math and biology homework Rachel had to complete. 

That is w + y. 

We know that w + y = 18 - x. 

That means that w + y = 18 - 6 = 12. 

That means that Rachel had to complete 12 pages of math and biology homework.