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.