Question

a-b=6,a=2b+7

Answer 1

First off, get the a over to the other side.
a - b = 6
2b - a = -7

Now you can add these two together to get
b = -1

Substitute the b back in
a -(-1) = 6
a = 5

To do it by substitution, you just put what your a value is back in the equation..
2b +7 - b = 6
b = -1

Answer 2

Since the question is

a - b = 6

and 'a' is equivalent to 2b + 7 , You must substitute

The new equation is now; 2b + 7 - b = 6

Combine like terms (2b - b)

Bring it down and you get b + 7 = 6

Subtract 7 from both sides

The answer is b = -1