How do i do this and solve it

Hey there! :D

Since the sides are equal in length, we know this is an isosceles triangle. It will have two equal angles. 

We already know the vertex is 80 degrees, so the bottom two angles will be equal in measure. All angles in a triangle equal 180 degrees

180-80= 100

100/2= 50 

x= 50

I hope this helps!
~kaikers

Two because two of one thing is two.