We can use such formula
F=am, where F is resultant force acting on book, a is acceleration and m is mass
Now we can substitute our data.
F=1.5*2
F=3
If we know that on the book is acting 4N and friction force and resultt is 3N we can create such equation
4N-Fr=3N /-4N
-Fr=3N-4N
-Fr=-1N /*(-1)
Fr=1N - its the result.