Which value of a from the set {16, 18, 20, 22} makes the equation
2(42 - a) = 48 true?

a is equivalent to 18.

2(42 - a) = 48
2(42) + 2(-a) = 48     *distributive property of multiplication
84 - 2a = 48
-2a = 48 - 84
-2a = -36
-2a/-2 = -36/-2
a = 18

to check:
2(42-a)   = 48
2(42-18) = 48
2(24)      = 48
48          = 48

The answer would be b ! explanation: the formula for sa *surface area * is 2×l×w+2×l×h+2×w×h so in this case l=4w=12h=8so you use the formula to solve it and it give you the answer of 352^cm^2