To evaluate #f(x + 1)# substitute #color(red)((x + 1))# for each occurrence of #color(red)(x)# in #f(x)# and calculate the result:
#f(color(red)(x)) = 3 - 1/2color(red)(x)# becomes:
#f(color(red)(x + 1)) = 3 - 1/2color(red)((x + 1))#
#f(color(red)(x + 1)) = 3 - 1/2x - (1/2 * 1)#
#f(color(red)(x + 1)) = 3 - 1/2x - 1/2#
#f(color(red)(x + 1)) = 3 - 1/2 - 1/2x#
#f(color(red)(x + 1)) = (2/2 * 3) - 1/2 - 1/2x#
#f(color(red)(x + 1)) = 6/2 - 1/2 - 1/2x#
#f(color(red)(x + 1)) = (6 - 1)/2 - 1/2x#
#f(color(red)(x + 1)) = 5/2 - 1/2x#
Or
#f(color(red)(x + 1)) = (5 - x)/2#