Official solution
Negative of (p \[\vee\] r) \[\Rightarrow\] (q \[\vee\] r)
\[ \equiv \] \[\sim\] ((p \[\vee\] r) \[\Rightarrow\] (q \[\vee\] r)) \[\equiv\] (p \[\vee\] r) \[ \wedge \] (\[\sim\] (q \[\vee\] r))
\[\equiv\] (p \[\vee\] r) \[\wedge\] (\[\sim\] q \[\wedge\] \[\sim\] r) \[\equiv\] (p \[\vee\] r) \[\wedge\] \[\sim\] r) \[\wedge\] \[\sim\] q
\[\equiv\] ((p \[\wedge\] \[\sim\] r)) \[\vee\] (r \[\wedge\] \[\sim\] r) \[\wedge\] \[\sim\] q \[\equiv\] (p \[\wedge\] \[\sim\] r) \[\wedge\] f) \[\wedge\] \[\sim\] q
\[\equiv\] (p \[\wedge\] \[\sim\] r) \[\wedge\] (\[\sim\] q) \[\equiv\] p \[\wedge\] \[\sim\] q \[\wedge\] \[\sim\] r