so, she has three boards, 36, 39 and 42 inches, hmmm but she wants the length or longer side to be 3.25 ft, hmmm 36 ÷ 12 is just 3, so that won't do, 42 ÷ 12 is 3.5 ft, nope not that one, 39 ÷ 12 is 3.25, well, that's our board.
so the width has to be smaller than the length, and it has to be a full board, so since we used the 39 incher already, smaller than 39 is 36, so we'll use the 36 incher for the width, that is, the 3 feet one.
From the 3rd board, Mia is going to have to make it match with the 36 incher, that means she needs to saw it off by 6 inches, 42 - 6 = 36, and 6 inches is just 6 ÷ 12 = 0.5 feet.
Check the picture below.