b = -1-7ab=−1−7a
Can be rewritten to be easier to understand.
[tex]b = -1-7ab\\-1 -7a = -1-7ab[/tex]
In addition you can say the following equation is valid due to the way your problem was given to you.
[tex]b = -1-7a[/tex]
Of course, you can simplify equation 2
[tex]-1-7a = -1-7ab\\-7a = -7ab\\a = ab\\1 = b\\b = 1\\[/tex]
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.
[tex]b = -1-7a[/tex]
[tex]1 = -1 -7a\\2 = -7a\\-\frac{2}{7} = a\\a = -\frac{2}{7}[/tex]
And there you go :)
