4a - 5b = 7
substract 4a from both sides
4a - 5b - 4a = 7 - 4a
positive 4a with negative 4a
5b = 7 - 4a
devide both sides by 5
[tex] \frac{5b}{5} = \frac{7 - 4a}{5} [/tex]
[tex]b = \frac{7 - 4a}{5} [/tex]
