We first identify the given values, AB = 137. With this information we can have an equation for any of the two unknown. Let's say A = 137/B.
The volume of the box would be V = LWH, where it becomes V = 3(A-6) (B-6) and we substitute the equation with our first one to make itÂ
V = 3(137/B-6)(B-6) and simplifying it we have, V= 519- 2466/B - 18B
Next we get its derivative so V' = -18 -2466/B^2 = 0 where we have B = √137 or 11.70.Â
In getting A, we substitute it A = 137/11.70 = 11.70. The values of A and B are the same, √137 or 11.70 inches.