Explanation:
when 2 angles in a triangle have the same size, this triangle is an isoceles triangle with both legs being equally long, and the side connecting the equal angles being the baseline.
in our example here
XZ is therefore the baseline.
and then XY and YZ are the equal legs :
13x - 21 = 8x - 6
5x - 21 = -6
5x = 15
x = 3
XY = 13×3 - 21 = 39 - 21 = 18
YZ = 8×3 - 6 = 24 - 6 = 18
XZ = 3 + 4 = 7