The parallelogram has base $6 \; \text{cm}$ and height $3 \; \text{cm}$, so area $6 \; \text{cm} \times 3 \; \text{cm} = 18\; \text{cm}^{2} $

Alternatively, notice that if you rearrange the parallelogram into a rectangle, you can tile the square with eight of them. Therefore the area of the parallelogram is $144 \div 8 = 18 \text{cm}^{2}$