Answer:

Step-by-step explanation:
So we have the formula:

And we want to make r the subject of the formula.
So, first, let's multiply both sides by (r-3). The right will cancel. So:

Distribute the t:

Subtract rt from both sides. The left cancels:

Factor out an r from the right:

Divide both sides by (1-t). The right cancels:

Simplify the negatives:

Flip:

And we're done!