that is a linear pair meaning that the 2 equations equal 180°.
when you set up your equation it will look like :
3x+31+2x-6=180
you will then combine the line terms
3x+2x=5x
31-6= 25
now put everything back together
5x+25=180
you have to get "x" by itself. You HAVE to use the opposite sign to bring it over the equal sign.
5x-25=180
-25 -25
5x=155
divide both of the sides by the number that is connected to the variable that you are trying to get by itself (in the case your variable is x)
5x ÷ 5 = x
155÷ 5 = 31
x=31