Ok, we can consider William's age as an , and the age of the father as , then taking into account the data of the problem, we expreed it mathematically as:

It is understood then that we have an equation system with two unknowns, of which we want to know the value of X, so to solve it we will use the Crámer rule for determinant (Spanish version). We proceed to rewrite the system and identify the coefficients:

For x, by the method of determinate we have:

solved with pleasure by @juancmz