Hi there!
This is an isocele triangle, which means that both sides "CA" and "CB" are equal and that both angles "A" and "B" are equal.
Since we know that angle "A" is equal to 50°, then angle "B" is also equal to 50°.
We also know that all 3 angles in a triangle are equal to 180°.
Since angle "A" + angle "B" are equal to 100°, angle "C" has to be equal to 80° (because 50° + 50° + 80° = 180°).
Now that we know that angle "C" is equal to 80°, all you need to do is write an equation with the parentheses equal to 80 and then solve it by isolating "y" :
80 = 5y + 10
Subtract 10 from each side of the equation
70 = 5y
Divide each side of the equation by 5
14 = y
There you go! I really hope this helped, if there's anything just let me know! :)