I think it's best to approach this problem more generally. Your method is ok since you verify your guess, but it wouldn't be too useful in a situation where the base is something like 6.1, and it can lead to carelessness as it did in this thread (resulting in an incomplete solution)!

Solve the polynomial

2r^2 + 2r + 5 = 89

2r^2 + 2r - 84 = 0

6 is a solution, but now we also see that -7 is!

(P.S. The three bars equals means, "is logically equivalent to" or "by definition." In most cases, like here for example, it's effectively the same as a regular equals. The three bars equals can almost* always be replaced by a regular equals, but the reverse cannot be done so often without making a few additional notes. For example, if you are calculating the time it takes for someone to run 10km at 15 km/hr, it wouldn't make sense to say the t (triple equals) 2/3hr, unless you clearly defined t using the given conditions, since time in general won't equal 2/3.)

*I'm covering myself incase someone decides to be exceedingly pedantic.