If you divide the first equation by i1 (in order to get Z), then the i1 on the first term on the right hand side will divide out, but you will have the expression i2/i1 on the second term. Use the second equation to solve for i2/i1 (easy to do since u2 = 0). Substitute that value into the first equation and simplify. You will then wind up with the required expression for Z.