Detemine the correct solution for 6x - 4 + 2x = 7x - 4

Here you go, I hope this helped you out

This is not a full question.  If you were given the amount at the begining of 1970 (we'll call it x) and the amount at the end of 1980 (we'll call it y), we should solve them accordingly:
y-x=? (calling the answer a)
a/y=%(not answer yet, call this b)
b+100=answer