Quadratic equation problem/optimization

"Sweet Harmony crafts has determined that when x hundred dulcimers are built, the average cost per dulcimer can be estimated by:

$\displaystyle C(x) = 0.1x^2 - 0.7x + 2.425$,

where C(x) is in hundreds of dollars. **What is the minimum average cost per dulcimer and how many dulcimers should be built in order to achieve that minimum?**"

Just need some guidance on how to interpret this; what I should be doing to find out the answers to the questions in bold. Thanks!