Firstly, look at the given area of the rectangle. As this is 20cm2, the length and width of the rectangle could be a combination of 20 and 1; 10 and 2 or 5 and 4. Any of thse options could be used to solved the problem, but for example using, 5 and 4:
Multiply 5 and 4 respectively by the scale factor of 3. This given 15 and 12. Multiplying these two numbers together gives a new area of the enlarged rectangle of 180cm2.