Let's make this into a system of equations. Our first equation is
River C + River D = $5,570$
Our second equation is
-River C + River D = $200$
We can find the length of River C by using the elimination method for a system of $2$ equations because River C in equation $2$ is negative and River C in equation $1$ is positive (postive + negative = elimination). We get:
River C + River D = $5,570$
+ -River C + River D = $200$
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2 * River D = $5,770$
To get rid of the $2$ we divide both sides by $2$ to get
River D Length = $2885$
Now we can plug River D's Length into the first equation to get
River C + $2885$ = $5,570$
We subtract $2885$ from both sides to get
River C Length = $2685$
So our answer's are River C = $2685$ & River D = $2885$!
NOTE: I was just reviewing this and realized I read the problem wrong I swaped the legnths for River C & D my bad. The real answers are:
River C = $2885$ & River D = $2685$
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+71 