You get rid of the denominator!
The reason you talk about a denominator is that your problem must contain a fraction.
So you have an equation containing a fraction, and the denominator contains a variable you may be trying to solve for. Multiply the whole equation by the denominator. That fraction disappears and the denominator is multiplied by other terms in the expression. Now the variable you want is not part of a fraction, so go on and solve. You may need to get rid of other fractions in the equation. Just multiply through by their denominators (one at a time is the safest way). Or you can multiply through by the LCM of a set of denominators to make the calculations easier.
If the variable you want is in the denominator and also elsewhere in other terms (but not in any denominators), you will encounter a quadratic or higher degree polynomials, and then you will need to resort to solving a quadratic, etc., and the solution may consist of several solutions for the variable.
When you arrive at one or more solutions, plug in each solution into the original equation to make sure you don't get anomalies, such as dividing by zero. Some questions contain a hidden trick that makes you think you have found a solution; but that solution is, in fact invalid because it creates an impossibility in the original equation. For example, the log of a quantity which evaluates to a negative value or zero; or the square root of a negative number.