We are give the equation of the perimeter of the triangle as follows:
2a + b = 15.7
where b represents the base.
Now, if we want to calculate the length of the base, all we have to do is isolate the b in one side of the equation as follows:
b = 15.7 - 2a
We know that a = 6.3 cm, therefore, the length of the base can be calculated as follows:
b = 15.7 - 2(6.3) = 3.1 cm