#f(x)# is literally another way of writing y so all you have to do is put in a value for #x#. For example, put in 0 for x: #f(0)=4-1/3(0+3)# and you get #f(0)=3#. Like I said before #f(x)# is really just y so plot #(0, 3)# since you plugged in 0 for x and found y to be 3