1. This is an isosceles triangle, so 2x = 50º. The sum of all the angles in a triangle must be 180º. These two bits of information are enough to allow you to calculate the value of y.
2. This is an isosceles triangle, with AB = AC. This allows you to find x and hence the size of BC.
3. Same as 2.
4.This is an equilateral triangle, so all the sides are the same length. Equate any two to find the value of x.
5. This is an equilateral triangle, so all the angles are equal to 60º.