transfer function of difference equation

yp(k+1) = (yp(k)yp(k-1)[yp(k)+2.5])/1+yp(k)^2+yp(k-1)^2 + u(k)

can someone please help me calculate the transfer function of the above in the form

H(z) = Y(z)/U(z) = b0 +b1 z^(-1) + ..... / 1 + a1*z^-1+.......

thank you