The number in the blank box should be 44 because it said that "3 times x and 4 times y is 44".
You need to multiply the first equation by 3:
3x + 3y = 39.
Then, subtract the first equation from the second equation:
3x - 3x = 0
4y - 3y = y
44 - 39 = 5
Your new equation should look like this: y = 5
Now, substitute y = 5 into the first original equation:
x + 5 = 13
x = 8
Ans. (x, y) = (8, 5)