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The four yellow triangles on the left all have the same area as the four yellow triangles on the right (a+b/2) - therefore, the two small green squares on the left have to have the same area as the large green square on the right.
The area of both large squares are equal; so the triangles with the square(s) must have a total area that is equal to each other.
length of a + length of b = length of c