Question 862389
Both of the other tutors started out fine but made errors.

There is a simple answer. 
What grade is your daughter in?
18 = (64 b)/((b-7) (b+7))

b=9 
the boat goes 9 mph 
One started out fine but made some simple errors
18 = (64 b)/(b^2-49)
18(b^2-49)-64b=0
18b^2-64b-49*18=0
18b^2-64b-882=0
using the quadratic equation
*[invoke quadratic "x", 18, -64, -882 ]

and factoring
 *[invoke change_this_name4450 18, -64, -882, "x"]