Answer:
√(205)/13
Explanation:
You want sec(θ) when cot(θ) = 13/6.
Trig relations
Some calculators can figure this directly as ...

Others need to make use of the trig relations ...
tan(θ) = 1/cot(θ)
sec(θ) = √(tan²(θ) +1)
or
sec(θ) = 1/cos(θ)
Application
The tangent of the angle is
tan(θ) = 1/cot(θ) = 1/(13/6) = 6/13.
Then the secant is ...
sec(θ) = √((6/13)² +1) = √(6² +13²)/13
sec(θ) = √205/13
A calculator can get there from ...
sec(θ) = 1/cos(arctan(6/13)) = √205/13