To solve this problem you must apply the proccedure show below:
1. You have the area of the larger square shown in the figure above, is:
A=x²
2. Then, you have the area of the smaller square shown in the figure above, is:
A=3²
A=9
3. Therefore, the expression for the remaining shaded area is:
A=x²-9
A=(x-3)(x+3)