Supplementary: Angles that add up to 180 degrees.
This means that angle 1 and angle 2 add up to 180 degrees. You can put them into an algebraic problem like I have in this (the image I have added), and solve for what x is. That is the key to finding out what angle 1 equals!
Once you have found x you can plug it into the equation you have for angles 1 and 2 to find if they equal 180.
Hope this helps!