lsqnonlin varible function to solve multiple equation

I have to solve bx=N{{3*m(x-a)+3*n(y-b)}*(x-a)/{(x-a)^2+(y-b)^2}^(5/2) -m/{(x-a)^2+(y-b)^2}^(3/2)} by=N{{3*m(x-a)+3*n(y-b)}*(x-a)/{(y-b)^2+(y-b)^2}^(5/2) -n/{(x-a)^2+(y-b)^2}^(3/2)} m^2+n^2=1

N is 19000000 bx,by,x,y is measured values.like,x and y in function a,b,m,n is the value that I have to find. Could you help me to find value with lsqnonlin function? I've been tried but it said use cell matrix but i don't now how to do Thank you...