let x be the width
then x + 3 is the length
2x + 2(x +3) =42
2x + 2x + 6 = 42 - applying distributive property
2x + 2x + 6 -6 = 42-6 apllying subtraction property of equality
4x/4 = 36/4 division property of equality
x = 9 cm width
x + 3 = 9 + 3=12 cm length
A = lw
A = 9(12)
A = 108 sq. cm.