I am just going to assume that the non-shaded box is a square, and tha its side lengths are equal to x.
A.
[tex]Area = 8^{2} - x^{2} [/tex]
This is because the area of the shaded is the area of the bigger square minus the area of the smaller.
B.
[tex]Area = 8^{2} - x^{2} \\ Area=8^{2}-3^{2} \\Area=55cm^{2} [/tex]
This is found by just plugging in the value of x into the previous expression.