You have to apply the superposition. You have a resistor R, a current generator that sources Ig amperes and a voltage generator that generates Vg volts.
We need to fix arbitrary the polarity of the voltage (or the sense of the current) on R. Let's suppose the "+" is on the side toward the current generator.
Now open the current generator (that is put Ig=0). In this case the voltage across the current generator will be Va=-Vg
Then reconnect the current generator and short the voltage generator (that is Vg=0). The voltage across the current generator will be Vb=R*Ig
Due to the superposition we can calculate the total voltage across the current generator as Va+Vb, that is -Vg+R*Ig
Numerically -1000+2*10 = -980 V