So there are 2 different particles. 1 mol of X produces 1 mol of Y and 2 moles of Z.
Kps = [Y] [Z]^2
We will call “s” (solubility) the molarity of X
So the molarity of Y+ is also “s” (same number)
And the molarity of Z is “2s” (twice as much)
Kps = s*(2s)^2 = s*4s^2=4s^3
If s is multiplied by 2:
Kps = 4*(2s)^3=4*2^3*s^3=4*8*s^3
So Kps is multiplied by 8.