#int pisinpix#
We bring out the constant #pi# outside the integral
#piintsinpix#
Now we can apply U-Substitution
#Let# #u=pix#
#piintsinu# #dx#
#(du)/dx=pi#
We want to get #du# by itself so we multiply both sides by #dx#.
#du=pi# #dx#
#color(blue)piintsinu# #color(blue)dx#
Since we have #color(blue)pi# #color(blue)dx# in the integral we can substitute it for #du#.
#intsinu# #du#
Now since everything is in terms of #du# you can integrate:
#intsinu# #du# #rArr# #-cosu+c#
Substitute #u# into the answer:
#-cospix+c#