there are a lot of different methods to solve this system of simultaneous equations. I'll demonstrate 2 of them:
First rearrange the 2nd equation!
1. multiply the 2nd equation by 4: and add columnwise:
. Plug in this result into any of the 2 equations and solve for e.
2. Solve the 2nd equation for e: e = 2f - 11 and substitute e by this term in the first equation:
Plug in the result for f into the 2nd equation.