Aren't both 5 and -5 the square roots of 25?

The square root of a positive number cannot be defined because we don't know if that number came by squaring -ve number or  +ve number. Therefore, we write +-5 for the square root of 25.

I hope this was helpful :)

Very helpful, thank you. Confirms that the error was indeed not mine, much to my surprise. Usually I am the one making the error, but in this case, my calculations matched that of the hints, and the only difference was that the hints said that one of the two values for the variable, y, could be ruled out due to it leading to the "false" conclusion that -5 = sq.rt 25 when in fact that is not a false statement.

Now that I know such an error can occur on Khan Academy, I will be prepared to report an error in Khan if I ever see one (after determining that the error wasn't mine, to the best of my ability).