And then this angle which is considered to be an exterier angle, so in this example y is an exterior angle in this example that is our exterior angle that is going to be supplementary to 180 minus a mius b so this angle plus 180-a-b is going to be equal to 180 so if you call this angle y you are going to have y plus 180 -a -b is equal to 180 you can subtract 180 from both side you can add a and b to both sides so +a +b +a +b I'm running out of space on the right hand side. and then you are left with - these cancel out - on the left hand side you are left with y on t he right hand side is equal to a plus b. so this is just a general property. you can you can just reason it through yourself just with the sum of the measures of a inside of of an the angles of an inside of a triangle add up to 180 degrees and then you have the supplementary angles right over here or you van just say look if I have the exterior angles right over here it is equal to the sum of the remote interior angles thats just a little terminology you'd see there so y is equal to a +b , 114 degree we've already shown to ourselves is equal to 64 + 50 degrees, anyways regardless of how we did it if we just reason i t out step by step or if we just knew this property from the get go if we know that y = 114 degrees and