When two straight lines cross, it is found that the angles opposite to each ate the same size. They are known as

The angles don't need to be lined up vertically, or have one over top another. You could have them laid out horizontally. All that matters is that they are opposite angles after an X shape has been formed by two crossing lines. Vertical angles are always congruent.

take the logarithm of both sides (the base of the log doesn't matter):

apply the exponent property - this says that :

distribute the log terms on both sides:

group like terms together:

divide through both sides by the coefficient on :

you can do some additional rewriting here using the properties of the logarithm to simplify things if you like: