The question asked you to find $\displaystyle \sin \theta$, not solve for $\displaystyle \theta$. You have found $\displaystyle \sin^2 \theta = \frac{4}{13} \Rightarrow \sin \theta = \pm \sqrt{\frac{4}{13}}$. Your job is to decide which of the + or - solutins to keep.
but in order to check if u have correct answer, if u try to substitute the theta value to the 1st given equation, it doesnt fit...
is there a problem ?
No, there is no problem. You are obviously making a mistake somewhere. But unless you show what you did and why "it doesnt fit ...." it is impossible to know what mistake yuo have made. My previous reply tells you what to do.