An isosceles triangle has two sides of equal length. Let the two unknown sides have measurement x.

perimeter= AB+BC+CA

20= x+ x+ 8

20= 2x + 8

2x = 20-8

2x= 12

x=6

So measure of side BC is 6.

perimeter= AB+BC+CA

20= x+ x+ 8

20= 2x + 8

2x = 20-8

2x= 12

x=6

So measure of side BC is 6.