(1) Usually when an exponent in an equation is of the form ax+b, it is going to help if you separate the expression into two parts. In your example, rewrite "25^(3r+3)" as the product of 25^(3r) and 25^3.
(2) The bases for the exponents are 125 and 25, which are both powers of 5. So rewrite the equation using 5 as the base for the exponents.
And now we have a problem. 5^(6r) is a positive number, so we can divide both sides of the equation by it to get
That is clearly false; that means the original equation has no solution.