Q: A kite $100 \ m$ above the ground moves horizontally at a speed of $8 \ m/s$. At what rate is the angle between the string and the horizontal, i.e. e the ground, decreasing when $200 \ m$ of string has been let out.

The only part I am having trouble with is forming a relationship between the angle $a$ and the horizontal displacement $x$. 

I figured that it has something to do with $tan$, but I have difficulty in differentiating an inverse $tan$ function (Which I have not learnt or will learn yet) after finding the relationship between to be $tan \ a=100/x$.

Any help would be appreciated,

Thank you.