Help please! I have choral practice...

This polygon is composed of a right triangle and a parallelogram.

The area of right angle :


The area of the parallelogram:


The area of the polygon is equal:

Therefore, the answer is:

Slice the picture in half, across the middle dotted line.

The top half is a triangle.  Its base is 15cm and its height is 8cm .
Area of a triangle is (1/2) (base x height) = (1/2) (120cm²) = 60 cm².

The bottom half is a parallelogram.  Its base is 15cm and its height is 5cm.
Area of a parallelogram is (base x height) = (15cm x 5cm) = 75 cm².

Add the two pieces together.  (60cm²) + (75 cm²) = 135 cm²

Breathe from the diaphragm, enunciate, and watch the conductor.