S= 6ab + 4a^3make b the subject of the formula

Solve for b:S = 4 a^3 + 6 a b
S = 4 a^3 + 6 a b is equivalent to 4 a^3 + 6 a b = S:4 a^3 + 6 a b = S
Subtract 4 a^3 from both sides:6 a b = S - 4 a^3
Divide both sides by 6 a: b = S/(6 a) - (2 a^2)/3