How do you use equations to solve problems about area of rectangles,
paralellograms , trapezoids and triangles

Each shape has a different formula for area. rectangles and parallelograms share the same formula, A=bh
Trapezoids formula is A= 1/2(b1 + b2)h
Triangles formula is A = 1/2bh
In all of these formulas, A means area, and h means the height of the shape. In the formulas for triangles and rectangles/parallelograms, b stands for the length of the base of the shape. 
In the trapezoids formula, b1 means base 1 and b2 means base two, as a trapezoid has two different bases.
After finding all of the variables for the formula of the shape you're solving for, plug all the numbers into your missing variables, and solve as normal.

D.). X=10. It is the same equation, only simplified.