2a(5-b)=3b+7
[open bracket]
10a - 2ab = 3b + 7
[solve for b, so we need to move all terms with b to the left]
[-3b on both sides]
10a - 2ab - 3b = 3b + 7 - 3b
10a - 2ab - 3b = 7
[move all those without b to the right]
[-10a on both side]
10a - 2ab- 3b - 10a = 7 - 10a
-2ab - 3b = 7 - 10a
[divide by -1 through to change b to be positive]
2ab + 3b = 10a - 7
[take b out as the common factor]
b(2a + 3) = 10a - 7
[divide by (2a+3) through]
b = (10a -7)/(2a+3)
