perimeter:
p = 2b + 2h = 68
b = 4h - 6 (per problem statement)
substituting for b in the perimeter equation:
p = 2(4h - 6) + 2h = 68
(2)(4h) - (2)(6) + 2h = 68
8h - 12 + 2h = 68
10h - 12 = 68
10h = 68 + 12
10h = 80
h = 80/10
h = 8
b = 4(8) - 6 = 32 - 6 = 26
p = 2(26) + 2(8)
p = 52 + 16 = 68 . . . [OK]
b = 26 cm
h = 8 cm