We can make two simple equations to express their ages to each other, then solve for one of the variables. Let n be Nojo's age and j be Jacob's age.
n(j) = 84
n - 5 = j
Use substitution to get only one variable in an equation.
n (n - 5) = 84
n^2 - 5n = 84
Since we have n to the power of 2, this equation has two possible answers, but since we are given four possible answer, just substitute them in the equation until one makes the left side equal 84.
n^2 - 5n = 84
12^2 - 5 (12) = 84
144 - 60 = 84
84 = 84
The answer is C = 12