csctheta=cotsquared theta-1
$\displaystyle cosec ( \theta ) = cot^2 (\theta ) - 1 \Rightarrow cosec ( \theta ) = cosec^2 (\theta ) - 2 $.
Substitute $\displaystyle x = cosec ( \theta )$, re-arrange and solve the quadratic equation for x.
Now use the value(s) of x to solve the equation $\displaystyle x = cosec ( \theta )$ for $\displaystyle \theta$.