a)  $P=a^2={\left(23cm\right)}^2=529c{m}^2$

b)  $P=a^2={\left(2,5dm\right)}^2=6,25d{m}^2$

c)  $P=a^2={\left(10,7m\right)}^2={114,49}^2$

d)  $P=a^2={\left(8\frac{1}{4}cm\right)}^2={\left(\frac{33}{4}cm\right)}^2=\frac{1089}{16}c{m}^2$