You could use this approach, assuming you're referring to the pixel resolution (the values stored in each pixel are not the area of the pixel):

Example:

30m pixel = 900 m^2

pixel count = 10,000

Area (m^2) = 10,000 * 900 = 9,000,000

Area (km^2) = 9,000,000/1,000,000 = 9 km^2

Thanks Michael

It is also necessary to take into account, that resolution is usually given for nadir, so you should make a correction according to the inclination of satellite look to the object...