The cosine of an angle is equal to the adjacent side divided by the hypotenuse. In a right triangle, the hypotenuse is the longest side. Its value is 13. It also follows that B is the adjacent side.
To solve for B,
B = 13 cos 22.6˚ = 12
The tangent of the angle is equal to the opposite side divided by the adjacent side. Since B is known, therefore the answer is
tan(22.6˚) = a/12