Question 993675
Here
L = 12 + 3H
P = 2L + 2H = 400
Substitute for L in the second equation and get
2(12 + 3H) + 2H = 400
24 + 6H + 2H = 400
8H + 24 = 400
8H = 376
H = 47
L = 12 + 3(47) = 153