b = -1-7ab=−1−7a
Can be rewritten to be easier to understand.
In addition you can say the following equation is valid due to the way your problem was given to you.
Of course, you can simplify equation 2
This ends up giving you b so you don't end up having to do any substitution in order to find b. Now substitute the value for b into either equation. I'm choosing the third because it's easiest.
And there you go :)