for the trapezoid/trapezium
the formula for finding the area of a trapezoid is A=(a+b)/2*h, and if you didnt know, a and b are always parallel.
for the parallelogram
it will always be b*h, this one is pretty easy to remember, also parallelograms include squares, rectangles, and anything with 4 sides.
the example
lets just say that we are missing a, which is the top one. now we have to fill in the missing little bit marked by a green square. this particular trapezoid is symmetrical, so we dont have to find the other side, but if it wasn´t, then you have to find the other side with the Pythagorean thoream. That is what we are going to do now. We have a and c and we need to find b in a²+b²=c², so plug in our values for a and c
(10)²+b²=(13)²
100+b²=169
now just isolate b and solve
b²=69
how to undo a square is by taking the square root of the other side of the equation
sqrt(69) is about 8.3
sorry this took me forever
