Step-by-step explanation:
length = 2×width + 6
84 = 2×length + 2×width
now using the first in the second equation :
84 = 2×(2×width + 6) + 2×width = 4×width + 12 + 2×width
84 = 6×width + 12
72 = 6×width
width = 12 cm
length = 2×width + 6 = 2×12 + 6 = 24 + 6 = 30 cm
