Let x = the length of each of the two equal sides The base is 12 inches less or x-12 /\ / \ x/ \x / \ /________\ x-12 left side + right side + base = perimeter x + x + x-12 = 39 3x - 12 = 39 3x = 51 x = 17 The two equal sides are 17 inches each The base is x-12 or 17-12 or 5 inches. Edwin