Please help me solve this.

a = v/t     Multiply both sides of the equation by 't

at = v      Divide both sides by 'a'

t = v/a

A=pi r^2     Divide both sides by pi

A/pi = r^2    Sqrt both sides

sqrt(A/pi) = r                                 Do you see how it is done?   Can you do the third one?

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.