When we draw sine graphs, we do not try to plot all the points but initially only important points.

In the graph #f(x)=sinx#, maximum and minimum value is taken as #1# (at #x# = #pi/2#, #(5pi)/2#, #(9pi)/2# ...) etc, and #-1# (at #x# = #(3pi)/2#, #(7pi)/2#, #(11pi)/2# ...).

Apart from that #0# value is taken at (#0#, #+-pi#, #=-2pi# and so on).

These form the important points.

In #f(x)=2sin(x/3)#, maximum value taken is #2#

at #x# = #(3pi)/2#, #(15pi)/2#, #(27pi)/2# ...) etc. and

minimum value taken is #-2#

at #x# = #(9pi)/2#, #(45pi)/2#, #(81pi)/2# ...) etc. and

#0# value is taken at (#0#, #+-3pi#, #+-6pi# and so on).

Note that these are all at #(3pi)/2# intervals and maximum and minimum value is #+-2#.