The 1/4 of the ellipse's arc length is known ($\displaystyle 3$ in this case) and so is the semi-major axis $\displaystyle a $. So, for each $\displaystyle a $ that would be plugged in, how would you get $\displaystyle b $? (Headbang)

Here is (the picture of) the equation:

http://img34.imageshack.us/img34/7319/giflatex7.gif