-3/4(1^2)(1+3) = -3/4(1)(4) = -3/4(4) = -3

При x = 1 1/3:

-3/4((4/3)^2)((4/3)+3) = -3/4(16/9)(25/3) = -3/4(400/27) = -1200/108 = -100/9