The side labeled "a" is made the longer side. These are the two congruent sides. So a > b.
We are told that the longer side is 6.3, so a = 6.3, meaning that
2a+b = 15.7
2(6.3)+b = 15.7
12.6+b = 15.7
b = 15.7-12.6
b = 3.1
The triangle has sides of: 6.3, 6.3, 3.1
The perimeter is 6.3+6.3+3.1 = 15.7
Going back to the question "which equation can be used to find the length of the base?", it sounds like either you were given a list of multiple choice answers, or you just fill in the blank. If multiple choice, then try to see which answer matches with what I wrote above. If fill in the blank, then I would just enter either 2(6.3)+b = 15.7 or 12.6+b = 15.7
