I think of a number multiply it by itself then add 6 to the result.

Unfortunately, you failed to include in here the amount that has to be paid by the family initially. However, as a general knowledge, the amount that has to be paid for a credit after n years in a simple interest can be calculated through the equation below,
                                  F = P  + Prn
where P is the initial value, r is the decimal equivalent of the given rate and n is the number of years. If the given simple interest rate is yearly then, the equation would become,
                                    F =  P x (1 + (0.12)(3/12)) = 1.03P