TeX source:
\begin{array}{rcl} \displaystyle\int\limits_0^3 2x \, dx &=& 2\cdot\left[\dfrac{x^2}{2}+c\right]_0^3 \cr\cr &=& 2\left(\left(\dfrac{3^2}{2}+c\right)-\left(\dfrac{0^2}{2}+c\right)\right) \cr\cr &=& 2\left(\dfrac{9}{2}+c-0-c\right) \cr\cr &=& 9 \end{array}