So far so good! Let's look at a harder example: 2a + b = 21
In this case we cannot just solve for a or b individually, as both variables appear in each equation. Instead we must rearrange an equation to find an unknown in terms of the other: b = 15  a
Now we can substitute this statement for b into the first equation: 2a + b = 21
This can now be tidied up, giving a correct answer for a: 2a  a = 21  15
Finally we can use any of the first 3 equations to find b: b = 15  6
