Question 1007823
No worries.  Remember that rate times time equals distance, or rt=d.
Let us call the rate in still water, r.
Let us call the rate of the current, c.
Then, from the facts in the problem we have
(r+c)(12) = 252  and
(r-c)(36) = 252
We can multiply these out and get
12r + 12c = 252  and
36r - 36c = 252
Now multiply the top one by three and add it to the second one...we get
36r - 36c = 252
+(36r + 36c = 756)
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72r = 1008
Now divide by 72 and get
r = 14 miles per hour
c can be found by substituting into the very first equation...
(14+c)(12)=252
Divide by 12 and get
14 + c = 21
c = 7 mph
Ta-daa!