I don't know any Lisp, but I would decode

    (= (* x (+ y z))
       (+ (* x z) (* y z)))

As

  x*(y + z) = x*z + y*z

e.g. (+ y z) means you add y and z with highest priority.

That is probably not a statement of equality, but a boolean test for it.

Well, technically it solves to x = z when y <> 0 and any x, z when y = 0 though I'm not sure if that's the original intention.