The answer is YZ.
since XY=XZ, XYZ is the isosceles and naturally angle Y = angle Z = 57 degree.
so in a traingle the longest side is on the opposite side of the biggest angle.
If the congruent sides measures 57, the angle opposite the base( angle X) would mean 180-57-57 = 66. So basically the side which is opposite to X is the longest and that side being YZ.