rotated straight line substending an angle at the x-y axis

Question

We have a straight line situated on the x-axis and it starts from the origin and its endpoints can be considered to be $(0,0)$ and $(d,0)$. We move this line through an angle $\theta$ and now we have its length $d_1$. We know the value of $\theta$ and also $d_1$. How can we proceed to find $d$? Making a triangle and doing $\cos d_1$ doesn't seem to give a correct result.