I'm going to assume you mean "u= 4i- 3j+ 5k".

A vector is "parallel to the xy plane" if and only if it is of the form xi+ yj+ 0k for some numbers x, y.

For such a vector to be perpendicular to 4i+ 3j+ 5k we must have which is the same as 4x= 3y. Choose any number you like for either x or y and solve for the other. Finally, find the length of that vector and divide by the length to get a unit vector.