Hey lauramorrison93.

Basically since you have three equations and four unknowns, you will be able to reduce your system to one un-known (or one as a function of the other).

Its like the situation in linear algebra where you reduce a matrix to get the simplest solution but in your case you have non-linear functions of the variables.

An example is say

x + y = 1

2x + y = 2

We let y = 2 - 2x and sub into 1 giving

x + 2 - 2x = 1 which gives solution x = 1

Its exactly the same thing except you have non linear terms.