If you look at the top sides and bottom sides of both figures, this means that they are the same or similar.
In this figure, they are similar because one side is half of the other.
So... divide the longer length in half and solve for x.
42 / 2 = 21
x - 1 = 21
x - 1 + 1 = 21 + 1
x = 22
Therefore, the answer is C.
Best of Luck!