Find the Coefficient of Correlation of the Line That Best $\{ ( 1,2 ) , ( 5,6 ) , ( 7,8 ) \}$

Find the coefficient of correlation of the line that best fits the data set.
​ $\{ ( 1,2 ) , ( 5,6 ) , ( 7,8 ) \}$ ​
Please give the answer to four decimal places if necessary.