First of all, let's single out the 12 feet long and the 5 feet wide. We need to figure out what 25% of the length (12 feet) is. Let's divide 12 by 4. We get 3. Now, let's remember that.
Second, we need to figure out what 20% of the width (5 feet) is. Let's divide 5 by 5. We get 1. Now, let's also remember that number.
Now, we must add the numbers we got to the original length and width. For 12, we got 3, and 12+3=15. For 5, we got 1, and 5+1=6.
Now that we have the new length (15) and the new width (6), we must multiply to find the new area. 15x6=90. Using the original length (12) and the original width (5) we will multiply to find the original area. 12x5=60.
Original Area: 60 feet.
Area after increasing length and width: 90 feet.
Change in area: 30 feet.
I hope this explanation helped you out so that you can do it yourself next time. If you need anymore help, feel free to message me.