If it didn't have that piece cut out on the bottom, it would be a rectangle that's 9cm wide and 5cm high. Its area would be 9x5= 45 cm².
But that piece is cut out of the bottom. The cut-out piece is 3cm wide and 3cm high, so its area is 3x3= 9 cm².
9 cm² have been cut out of the 45 cm² of the complete big rectangle. So the remaining area of the figure is 45 - 9 = 36 cm² .