Squaring both sides of your equation and coalescing given parameters produces a simplified equation of the form
N^(b+c) + k1*N^b + k2 = 0
where k1 and k2 are known. (The N^c term that would have been there has disappeared since you set sigmaM to zero.) If 'solve' cannot find an explicit solution for this, you will have to do it numerically, say, with 'fzero'.
By the way, your value for exponent 'c' looks like a suspiciously large negative quantity. Don't you mean