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You may delete entry soln[N] of your dict using del soln[N] :

sage: b,c,N = var('b c N')
sage: soln = {}; # defining dictionary
....: soln[b] = 0.5
....: soln[c] = 0.2
....: soln[N] = 6;
....: 
sage: del soln[N]
sage: soln
{b: 0.500000000000000, c: 0.200000000000000}