What is the area of a rectangle with the height of 25mm, and the length of 12mm? after you get the product, turn it into feet.

Area = (length) x (width) = (25mm x 12mm) = 300 square millimeters

You can't convert 'square millimeters' to feet.  One is a unit of area,
but the other is a unit of length.

You can convert square millimeters to square feet, though.

(300 mm²) x (1 ft / 304.8 mm)²  =  0.00323 square foot (rounded)