is the same thing here as the other triangles, the triangle below is an equilateral, the one above is an isosceles.
the vertex of the isosceles is 40°, so the twins angle at the base are (180 - 40)/2 = 70° = x each.
since the sides in the equilateral are all the same length, then 2y + 10 = x - y, but we already know what "x" is, so
[tex]\bf 2y+10=x-y\implies 2y+10=\stackrel{x}{70}-y\implies 3y=60 \\\\\\ y=\cfrac{60}{3}\implies y=20[/tex]
