If this is -7/(3n)=-42, we can cancel the two minuses: 7/(3n)=42; divide each side by 7: 1/(3n)=6; now invert both sides: 3n/1=3n=1/6, so, dividing through by 3: n=1/18.
If this is -(7/3)n=-42, we cancel the minuses: (7/3)n=42; divide through by 7: n/3=6; multiply through by 3: n=18.
So the answer depends on interpretation of the question. In other words, is n in the numerator or denominator?