I have given the half angle formula.
If 2θ = 3π/4, half of that angle is θ = 3π/8 which is the required angle.
I have observed that you are not following our instructions. Every time you expect the complete solution. It is not good for any student who wants to study the subject.
In my post, you have to find the value of tan(3π/4), simplify the expression, solve the quadratic equation to get tanθ, i.e. tan(3π/8).
Can you try now?