its right because
6 x 5 is the area of the leftmost rectangle
4 x 5 is the area of the bottom rectangle
3 x 5 is the area of the rightmost rectangle
5 x 5 is the area of the slanted square on the top
the last term is the area of the trapezoid, which is (base1 + base2) x height /2
they split the trapezoid thing into a square and a triangle, which gets you the same answer
they multiplied it by 2 because there are 2 trapezoids