A box is measured at 3 ft by 1 ft. Find the sine of the angle that the diagonal of the box makes with the longest side.
A box has 3 dimensions SO I assume you are talking about a rectangle.
Draw the rectangle and put the diagonal and the angle that is spoken of on it.
Find the length of the diagonal using pythagoras's theorum.
Sine of the angle is the opposite side over the hypotenuse. Just read it off your diagram.