Theresa is 14 years younger than her brother. together, their total age in years is 35.

Let :
T = Theresa age
H = her brother
We know :
T = H - 14
T + H = 35
Substitute T
H -14 + H = 35
2H - 14 = 35
2H = 49
H = 49/2 = 24.5
Since we know H = 24.5 we then can substitute it to either equation.
T = H - 14
T = 24.5 - 14
T = 10.5
Theresa is 10.5 years old
Her brother is 24.5 years old