Explanation:
We have the formula for the surface area of a rectangular box given by:
S = 4LW + 2WH
Substituting the given values, we get:
138 = 4(9)(3) + 2(3)H
Simplifying, we get:
138 = 108 + 6H
6H = 138 - 108
6H = 30
H = 30/6
H = 5
Therefore, the value of the unknown variable H is 5.