Solving multiple knapsack in C++

I have n nodes to be made into m clusters of any size, provided at least one point in each cluster. I have an array P[n] of population of each node i.e P[i]. I also have a distance matrix d[n][n] where d[j][k] is the distance between node j and node k. The objectives are the following:

             minimise( d[j][k]*x[i][j]*x[i][k] where i running from 1 to m)

          and

             t1 <= P[k]*x[i][k] <= t2

where x[i][k] = {0,1}, whether node k is present in cluster i or not. t1 and t2 are any integers where t1

I know this is a variation of Generalized assignment problem, but I'm new to optimization. Please help me with the C++ implementation of the solution.

Apologies for the math symbols. Stackoverflow doesn't have a math library. Thanks!