So far the area was the answer. Now it's a GIVEN, and a side is hiding. Working backwards is division: a rectangle's width is $w = \dfrac{A}{l}$; a triangle's height is $h = \dfrac{2A}{b}$, the ½ un-halving into a 2 on top, exactly as last lesson promised.

Circles add one more move. $A = \pi r^2$ reversed gives $r = \sqrt{\dfrac{A}{\pi}}$ — r equals the square root of A over pi: divide the $\pi$ away first, then un-square with the root. Dividing by $\pi$ leaves $r^2$, and $r^2$ is not yet $r$ — stopping there is this lesson's favourite trap. The draws are kind: every root lands on a perfect square.