You have 2 triangles here making up this single quadrilateral. One of the triangles is a 30-60-90 and the other one is a 45-45-90. These 2 triangles share a hypotenuse. We need the length of that hypotenuse if we are to find the missing side x. The "lower" triangle is the 30-60-90 and the side we are given is across from the 30 degree angle. In the Pythagorean triple for a 30-60-90, the side across from the 30 degree angle is x, the side across from the 60 degree angle is

, and the hypotenuse is 2x. So we need to find x. If

then two times x is equal to

which is equal to

. So the length of the hypotenuse is

. The Pythagorean triple for a 45-45-90 is

with

as the length of the hypotenuse in a 45-45-90. If the length of the hypotenuse is

, then we need to solve for x.

. We solve for x by dividing by the square root of 2, like this:

. The only way to solve this is to multiply by

. Doing this step by step we have

. Multiply straight across the top and straight across the bottom to get

which simplifies by reduction to

. First choice above.