= Solution
Use the two <derived inference rules> whose complete <proofs> were supplied in (d). The following <logical derivation> has \b[no nested hypothetical subproofs]:
``
1 P → Q premise
2 Q → R premise
3 P → (Q → R) → creation from 2
4 (P → Q) → (P → R) → distribution from 3
5 P → R →-elim, 1 and 4
``
The creation instance takes $\phi=P$ and $\psi=(Q\to R)$. The distribution instance takes $(\phi,\psi,\rho)=(P,Q,R)$. The metarules are justified in (d); this requested flat use does not secretly invoke <implication introduction rule> in one of its own lines.
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