9
To balance the equation, there must be an equal number of Xs and Ys on each side. Since on the left side there are 2 Ys, and on the right side there are 3, you must put a coefficient of 3 on the left and 2 on the right to get the least common multiple of 6. Since putting 2 as the coefficient on the product makes a total of 4 Xs on the right, you must have a coefficient of 4 on the X product.
Thus, 4 + 3 + 2 = 9.
