We are given that the angle a is the right angle. So let us work from this.
ab = 12 (the vertical side of the triangle)
bc = 13 (which if drawn can be clearly observed to be the hypotenuse) = the side opposite to angle a
ca = 5 (the horizontal side of the triangle)
Since we are to find for the cosine ratio of angle c or angle θ, therefore:
cos θ = adjacent side / hypotenuse
cos θ = ca / bc
cos θ = 5 / 13
Check out the attached image below for the illustration of the triangle.
