Do you have more questions to it? Or maybe your teacher wants you to graph it?

If you want to graph it you need to find at least 2 points first. For example

f(x)=3x+3 is the same as y=3x+3

Pick any two numbers to "plug in" for "x" to get "y" just like this:

for x=1, y=3(1)+3=6   1st point is (1,6)

for x=2, y=3(2)+3=9  2nd point is (2,9)

(you can pick whatever you want for "x" )

now when you have to points you can put them on the graph and draw a line thru them. This will be a line with a linear equation f(x)= 3x+3

f(x)=3x+3 is not an equation. It's a line with the slope 3 and an Y-intercept of 3.