Because of the double negation (no + unless) I would first try to paraphrase the sentence before trying to translate it into predicate logic. All the following have the same meaning:
No dolphin sings unless it jumps
All dolphins don't sing unless they jump
All dolphins don't sing if they don't jump
All dolphins jump if they sing
The last one is easiest to translate. The translation is:
∀x(Kx → (Gx → Fx))
Which is equivalent to:
∼∃x∼(Kx → (Gx → Fx))
And thus:
∼∃x(Kx ∧ ∼(Gx → Fx))
