The Consecutive Interior Angles Theorem basically states that two interior angles (neither adjacent nor opposite) are supplementary, which means they add up to 180 degrees. That means that ∠2+∠4=180. ∠2=2x+10 and ∠4=4x=80. Using the Substitution Property, we get 2x+10+4x+80=180. Do some fun algebra:
6x+90=180
6x=90
x=15
Remember and know your properties and theorems. They will help you and will make proofs a LOT easier (and a LOT more fun).
