Question

Find Y of the polygon

Answer 1

I that 'y' is 45 degrees because of alternate angles (I think its called).

If you look at the large triangle on the left where the corner angles are 35 and 80 and 20+an unknown angle. We can work out the missing angle by doing 180-80-35-20= 45. 

And so if we use the fact that alternate angle must be equal then y= 45 degrees (I think!)

Answer 2

180° - 35° - 80° = 65° (sum of ∠s in a ∆)
65° - 20° = 45°
y = 45° (alt. ∠s)