The function #y=(x+1)(x-2)(x-4)# cuts #x#-axis (i.e. the line #y=0#) at three points,
Where #x+1=0# or #x=-1# and
#x-2=0# or #x=2# and at #x-4=0# or #x=4#
Now, these three points #(-1,0)#, #(2,0# and #(4,0)# divide #x#-axis in four parts
(1) In segment #x<-1#, all the three terms of #y# are negative and hence product is negative and the curve is below #x#-axis.
(2) In segment #-1 < x < 2#, while first term is positive, other two terms are negative. Hence product is positive and the curve is above #x#-axis. Note that it crossed #x#-axis at #x=-1#.
(3) In segment #2 < x < 4# while first two terms are positive, the third term is negative, hence product is negative and the curve is below #x#-axis. Note that it crossed #x#-axis at #x=2#.
(4) In segment #x>4# all the terms are positive, hence product is positive and the curve is above #x#-axis. Note that it crossed #x#-axis at #x=4#.
One can also put some other values of #x# now in different segments and complete the graph.
It appears as shown below.
graph{(x+1)(x-2)(x-4) [-20, 20, -10, 10]}
