secθ=3
1/secθ = 1/3
cosθ = 1/3
If you're not doing extension 1 maths, then the solution via general formula (the final answer 1729 posted) will probably not make sense to you. Depending on the domain, just solve θ like a normal trig equation. Assuming they only want the answer for the first quadrant, it would be
θ = cos^-1(1/3)
θ = 70 degrees, 32 minutes or 1.23... radians