sin^-1(1/√26) is the same thing as saying sin(θ) = 1/√26. sin^-1 determines an angle measure. Draw a right triangle such that sin(θ) = 1/√26, this means that the adjacent side is 5. You have a right triangle whose sides are 1, 5, and √26 (hypotenuse). You're being asked to find sin(2θ).