The area formula for a parallelogram is the same as it is for a square or a rectangle. It's A = b*h. They give you the area and the height, and x is part of the radicand for the base. Set it up like this:
. Divide both sides by 7 to get
. Now, the opposite of a square root is squaring, so let's undo that square root by squaring. But because this is an equation, if you square the right you also have to square the left.
. Squaring that square root undoes the square root, so now what we have is 36 = 6 + x, and 30 = x. There you go!