You can't get there from here

NP-complete

Many smart people have worked very hard on solving the knapsack problem
Also on a lot of not-obviously-related problems

        Bin packing
        Clique                  
        Dominating set
        Exact cover by 3-sets
        Graph 3-colorability
        Hamiltonian cycle
        Integer linear programming
        Longest path
        Minimum broadcast time
        Minimum cover
        Multiprocessor scheduling
        Partition               
        Partition into cliques
        Precedence-constrained scheduling
        Regex matching with backreferences
        Satisfiability
        Subgraph isomorphism
        Subset-sum
        3-dimensional matching
        Travelling salesman
        Vertex cover
        ... (hundreds more) ...

Nobody knows an efficient algorithm for any of these
But an efficient algorithm for "Knapsack" could be turned into an efficient algorithm for all of them
And it could be turned into an efficient algorithm for all problems in NP!
We say that Knapsack is NP-Complete
So are all those other problems, it turns out