Calculating Resistor Value in Voltage Divider
Try this :
$$V_o = \frac{R_2}{R_1 + R_2}*V_i$$
$$V_o*(R_1 + R_2) = R_2 * V_1$$
$$V_o*R_1 = R_2*(V_i-V_o)$$
$$R_2 = \frac{V_o*R_1}{V_i-V_o}$$



By the way, the output voltage is exactly the half of the input. This said, both resistor has to be the same. This statement isn't true if a load is apply at the output.