This isosceles triangle has two sides of equal length, a, that are longer than the length of the base, b. the perimeter of the triangle is 15.7 centimeters. the equation 2a + b  = 15.7 models this information. if one of the longer sides is 6.3 centimeters, which equation can be used to find the length of the base?

B = base length
a = length of the other two equal sides

The value of 'a' is larger than the value of b. This is because the base is shorter than the other two equal sides. We're told that "one of the longer sides is 6.3 cm", meaning that a = 6.3

Replace every copy of 'a' in the given equation with 6.3; isolate b
2*a + b = 15.7
2*6.3 + b = 15.7
12.6 + b = 15.7
12.6 + b - 12.6 = 15.7 - 12.6 ... see note below
b = 3.1

note: I subtracted 12.6 from both sides. This is using the subtraction property of equality.