Evaluate: cos(2arctan(1/3))

i used the double angle identity cos(2x) = cos^2x-sin^2x
cos(2x)= (3/sqrt(10))^2- (1/sqrt(10))^2 = 2/25
2/25 was my answer to this problem but the correct answer was 4/5
how is the correct answer 4/5? ? i thought i did everything right

The boiling point of water at an elevation of 0 feet is 212 degrees fahrenheit (°f). for every 1,000 feet of increase in elevation, the boiling point of water decreases by about 2°f. which of the following represents this relationship if b is the boiling point of water at an elevation of e thousand feet? a) e = 2b - 212 b) b = 2e - 212 c) e = -2b + 212 d) b = -2e + 212