The trapezoid's area can be understood as triangle a + triangle b. So, we write these triangles as an expression, and, since we cannot solve with no values, we simplify to prove the area of the trapezoid.
A triangle's area is 1/2bh. In their case, b is represented as b or a depending on the triangle.
t = 1/2ah + 1/2bh
Now, since both expressions use the terms 1/2 and h, distributed onto a second variable, it can be rewritten as a single expression where both are distributed onto both variables at the same time. This is because both terms being multiplied by something separately is the same as combining both terms and multiplying them both at the same time.
t = 1/2(a)h + 1/2(b)h
t = 1/2(a + b)h