he is correct
basically you have
h times base1+h times bsae 2 all divided by 2
that equals
(hb1+hb2)/2
if we undistribute h using distributive property which is ab+ac=a(b+c)
(h)(b1+b2)/2
if you split the trapezoid in half with a horizontal line that passes through the halfway point of the height and take those two shapes and put them side by side and cut the triangles off the ends and add them, you will see why this is true
the reason is becuase you can get to the area formula from that is height times (b1+b2)/2
