Answer: H = (A-2LW)/(2L + 2W)
==============================================
Work Shown:
L = length
W = width
H = height
A = surface area of a prism
A = 2(LW + HL + HW)
A = 2LW + 2HL + 2HW
A-2LW = 2HL + 2HW
2HL + 2HW = A-2LW
H(2L + 2W) = A-2LW
H = (A-2LW)/(2L + 2W)
Feel free to convert each letter to lowercase. I made them uppercase because lowercase L looks either like the number 1 or the uppercase letter i, so I wanted to avoid potential mix-ups.