You have to find the GCF of these numbers.
The factors of 64 are 1, 2, 4, 8, 16, 32, and 64, and the factors of 56 are 1, 2, 4, 7, 8, 13, 28, and 56.
The highest common factor is 8, so now let's divided.
64/8=8
56/8=7
Now......
64 + 56 = 8•8+8•7 aka 8(8+7) = 8(15) = 120
120 is your answer ;)