All "solve for" means is get the variable by itself on one side of the equal sign.
\(a=\frac{v}{t}\)
Multiply both sides of the equation by t.
\(ta=v\)
Divide both sides of the equation by a.
\(t=\frac{v}{a}\)
Hey! time = velocity / acceleration
\(A = \pi r^2\)
Divide both sides of the equation by pi.
\(\frac{A}{\pi} = r^2\)
Take the + square root of both sides.
\(\sqrt{\frac{A}{\pi}} = r\)
radius = the square root of (area / pi)
The third one is exactly the same as the second one just with different letters.