Pg 517 (449) - Q2
If you know that 2 sides of the triangle have the same length, and
the angle across from one of them is 60, then you know that the angle across
from the other side is also 60. The other angle of the triangle must
also be 60, which means that this is an
equilateral triangle with same-length sides all equal to 15.
Its perimeter, then, would be 3*15 = 45, which is B.