$$x^2 + xy + yz + xz = 30$$
$$y^2 + xy + yz + xz = 15$$
$$z^2 + xy + yz + xz = 18$$
I have tried manipulating the expressions, the identity of $(x+y+z)^2$ but to no avail.
Along with the answer, it would be great if you can explain the approach and thought process involved in dealing with such kind of questions.