Since the isosceles triangle has AB=AC, and we're given
AB=21x-14
AC=14x+7
we can equate
AB=AC
21x-14=14x+7
Solve for x
21x-14x = 7+14
7x=21
x=3
=> AB=21x-14=63-14=49 units
=> AC=14x+7=42+7=49 units
=> BC=35x-14 = 105-14=91 units
and since 49+49>91 units, the lengths satisfy the triangle inequality.
