Which calculation gives the area of this shape?

Cut along the dashed line: upper column 3 cm × 5 cm and lower bar 8 cm × 3 cm. Area = 15 + 24 = 39 sq cm. '$3\times 8+8\times 3$' counts the $3\times 3$ corner twice; '$8\times 8-3\times 5$' removes the wrong size (the missing corner is $5\times 5$).