This will not be linear.....thus any regession type equation will not perfectly fit the data...I used this app to create an (approximate) equation :   http://www.alcula.com/calculators/statistics/linear-regression/

This equation gets close

 y=1400+162x   where x is the bridge length and y is the cost of painting

thanks CPhil!  

First State Bridge-Painting Costs

$\begin{array}{|rcll|} \hline \text{Painting Costs } = 1000\cdot \left[ -24 + \frac{351}{6} \cdot \left(\frac{\text{Length}}{100}\right) - 19 \cdot \left(\frac{\text{Length}}{100}\right)^2 + \frac{15}{6} \cdot \left(\frac{\text{Length}}{100}\right)^3 \right] \\ \hline \end{array}$