"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!