Let M = Marty's age, and W = Warren's age
M+3 = 6W
Solve for M: M = 6W - 3 {Eqn 1}
M + W > 11 {Eqn 2}
Now you have two equations (Eqn 1 & 2}, and two unknowns (M, W)
There are a couple of ways to proceed. I recommend rewriting Eqn 2 as M + W = 11. Now you can easily use substitution. Solving for M yields M = 11-W. Now plug this back into Eqn 1:
11-W + 3 = 6W
14 = 7W
W =2
