3) 6 2/5 * 1 9/16 - (2 1/4)^2 = 32/5 * 25/16 - (9/4)^2 = 10/1 - 81/16 = -41/8 = -5,125
4) (2 7/10 - 1 8/15)^2 * 9/14 = (27/10 - 23/15)^2 * 9/14 = 49/36 * 9/14 = 7/8 = 0,875
