Help with a Trigonometry Problem?

Question

Could someone explain to me how they got this answer? Help would be much appreciated! Thanks.

Answer 1

cosθ=3x

Using pythogorus trigonometric rule,

tan^2θ=sec^2θ-1

Here secθ=1/cosθ

tan^2θ=(1/cos^2θ)-1

            =(1/(3x)^2)-1

            =(1/9x^2)-1

Therefore option B is correct