Current analysis with 2 voltage sources
recognize that the two voltage sources are not acting in the same direction.so you need find the resultant voltage in that circuit which is 50v-12.5v=37.5V and use the formula you wrote above to find I=37.5v/Rtotal = 37.5/15 = 2.5A. The voltage across the 7 ohm resistor is gotten by applying ohms law: v=IR = (2.5)(7)= 17.5v