If the object distance in the following picture were 8.0 m and the image distance were 2.0 m, what is the focal length? Answer options: 0.63m or 1.6m

Aresistor with an unknown resistance is connected in parallel to a 9 ω resistor. when both resistors are connected in parallel to an emf source of 21 v, the current through the unknown resistor is measured with an ammeter to be 4 a. what is the resistance of the unknown resistor? answer in units of ω.