All angles in the triangle add up to 180 degrees. So,
(x+15) + (3x-35) + 4x = 180
8x - 20 = 180
8x = 200
x = 25
First angle : x+15 = 25+15 = 40 degrees
Second angle: 3x-35 = 3 * 25 - 35 = 40 degrees
Third angle: 4x = 4 * 25 = 100 degrees.
It is an isosceles triangle since it has two angles (the first end the second one) equal to each other.
Hope that helps :)
