I for one have never heard of Tomassi much less his textbook (I taught logic many times).
It would be shear accident if anyone here knows the notation used in a particular text.
So if you want help with this, you are going have to type out the list of rules and notations used in that text.
Assuming that : means equivalent then No 3 can be solved in the following way:
P v (Q v R) = (P v Q)v R =.................................. ( by associativity)
=(Q v P)v R = .................................................. ..(By commutativity)
=Q v (P v R) .................................................. ......(BY associativity again).
Do you want the rest of the problems solved in this way??