Question 391902
Let,
S = Steve's age today
T = Teresa's age today

After four years their ages will be-
Steve : S + 4
Teresa: T + 4

Now let's write their relationships
Today        : S = 3T             ....(I)
After 4 years: S + 4 = 2(T + 4)   ....(II)

Let's substitute S = 3T from equation (I) into equation (II)
3T + 4  = 2(T + 4)
3T + 4  = 2T + 8
3T - 2T = 8 - 4
T = 4 years ... this is Tersa's age today
So Steve's age must be 3T = 3 x 4 = 12 years