【Question 10】
A square steel plate, after cutting off a rectangle 3 cm wide, and then cutting off another rectangle 5 cm wide (as shown in the figure), the area is reduced by 81 square cm compared to the original square. The original area of the square is $\qquad$ square cm.
【Analysis and Solution】
(Method One)
Let the side length of the original square be $x$ cm;
$$
x^{2}-(x-3)(x-5)=81 \text {; }
$$

Solving for $x$ gives $x=12$;
The side length of the original square is 12 cm;
The area of the original square is $12^{2}=144$ square cm.
(Method Two)
The area of $B$ is $3 \times 5=15$ square cm;
The side length of the original square is $(81+15) \div(3+5)=12$ cm; the area of the original square is $12^{2}=144$ square cm.