Question

Rearrange the formula S = a (a - r) for r.

Answer 1

I'm assuming you're trying to solve for r.
First divide a to both sides
Now you have $\frac{S}{a}$ = a - r
Subtract a to both sides.
And then divide by -1 to both sides.
Now your equation is -$\frac{S}{a}$ + a = r

Answer 2

Expand: a^2 - ar

S= a^2 -ar

S+ ar = a^2

Ar = a^2 - S

R = a^2 - S/ a