You can identify this figure as a square, which means all the side lengths have equal values.

$x^{2} = 225$
Solve for $x$.

$x=\sqrt{(225)}$

$\text{Area = (side)}^{2}$

$121 \text{ cm}^{2}$

$\text{Area = length (width)}$

$100 = 5w$
Solve for $w$.

See (a).