Solve the equation for b, so you need to isolate/get "b" by itself on one side of the equation.
[you can solve this different ways]
6b + 2a - 4 = 2b + 3a First subtract 2b on both sides
4b + 2a - 4 = 3a Next subtract 2a on both sides
4b - 4 = a Then add 4 on both sides
4b = a + 4 Finally divide 4 on both sides