Im having I little bit of trouble- I know i have to do substitution though

Question

Bored to the point of distraction, Marty’s mind wanders. Across one edge of his paper he scratches out the following: “The product of my age and my sister’s age is four less than six times the sum of our ages.” If Marty is four years older than his sister, how old is each? Show your work by writing and solving the equation Marty wrote about on his paper.

Answer 1

let M = Marty's age; let S = Marty's sister's ages; then:MS = 6(M + S) - 4; also: M = S + 4

solving: substituting second equation into first equation gives: (S + 4)S = 6(S + 4 + S) - 4

in which case: S² + 4S = 6(2S + 4) - 4.

or: S² + 4S = 12S + 24 - 4; or: S² + 4S = 12S + 20; or: S² + 4S - 12S - 20 = 0

or: S² - 8S - 20 = 0; or (factoring): (S - 10)(S + 2) = 0; or: S = 10; M = 14 (note: S > 0)