Kara has 21 collectible buttons to sell. she sells the small buttons for $3 and the large buttons for $4, and earns a total of $68. if the small buttons are represented by x and the large buttons are represented by y, which of the following systems of equations can be used to calculate the number of large and small buttons sold? x y = 21, 3x 4y = 68 x y = 68, 3x 4y = 21 x y = 21, 4x 3y = 68 x y = 68, 4x 3y = 21