GCF means greatest common factor. The factors are prime numbers that the number can divide without a remainder.
The GCF of 51 is 3 * 17
The GCF of 34 is 2 * 17
The distributive property states that
A(B + C) = AB + AC
The given expression is
34 +51
It becomes
(2 * 17) + (3 * 17)
Since 17 is common, we can factorise it. It becomes
17(2 + 3)
= 17 * 5
= 85