The forces acting on the block are (1) its weight (vertically downwards), (2) the tension T in the string (at an angle 25º to the plane), (3) the normal force N supporting the block on the plane (acting perpendicular to the plane because we're assuming there's no friction).

To calculate T, resolve the forces in the direction of the slope of the plane. Then the effect of N is eliminated, the weight makes an angle 50º (not 60º!) to this direction, and the tension makes an angle 25º. So the equation is .