Need. Help lattice proof

[Cratylus]

Theorem is from Pinters A Book of set theory


Attemped proof
by 4.36 x<(x v y )[MATH]\land[/MATH] z and y< (x v y )[MATH]\land[/MATH] z
so by 4.37
x v y ≤ (x vy)[MATH]\land[/MATH] z
But by 4.42 if x≤ z; x v z=z
then ?
Help