Determining Line of best fit

I have to figure out the line of best fit for children's books, the following data is given.

Recommended Age Leve - Mean Word Length

4 - 3.5

6 - 5.5

5 - 4.6

6 - 5.0

7 - 5.2

9 - 6.5

8 - 6.1

5 - 4.9

So i went ahead and made a table for x, y, x*y, x2 and y2 then i imputed the following data. Does this look anywhere close to being right?

Slope = n(Sigma(xy)) - (Sigma(x)) / n(Sigma(x2)) - (Sigma(x))2

= 8(268.2) - (50)(41.3) / 8(332) - (50)2

= 2145.6 - 2065 / 2656 - 2500

= 80.6 / 156

= 0.52

Intercept = Sigma(y) - m(Sigma(x)) / n

= 41.3 - 0.52(50) / 8

= 1.9125

y = mx + b

y = 0.52x + 1.91

then looking at this if i go to figure out what the average word length is for a 12 year-old i get 4.33... that's even less then what's recommended for a 6 year old?!?!?! Is there somewhere in here where i'm going wrong?