Question 635210
Distance(d) equals Rate(r) times Time(t) or d=rt; r=d/t and t=d/r

Let r=Anna's rowing speed in still water
Then r+3=Anna's rowing speed downstream
and
r-3=Anna's rowing speed upstream

Time required to row downstream=216/(r+3)
Time required to row upstream=216/(r-3) and we are told that this took 18 hours longer, sooooooo:
216/(r+3) +18=216/(r-3)  multiply each term by (r+3)(r-3)
216(r-3)+18(r+3)(r-3)=216(r+3)  next divide each term by 18 and simplify
12r-36+r^2-9=12r+36 collect like terms
r^2-81=0
r=+or-9  ----    -9 is no good; can't have negative rowing speeds
so
r= 9 mph Anna's roving speed in still water

CK

216/12 + 18=216/6
18+18=36
36=36

Hope this helps---ptaylor