Use a graphing calculator to approximate to 2 decimal places any relative minimum or maximum values of the function.
y=-9x^2+4x
Okay, you are looking for a screen (on my calculator called "Y=") where you can input the function. It will look something like this:
y1 =
y2 =
y3 =
(etc)
Move the cursor to y1 and input your equation.
Then you want to move to the "graph" screen. You'll get something like the graph below.
Now, you'll have a bunch of function keys (F1, F2, etc.) You are looking for the "Math" menu. (On my calculator its F5.) It has a drop-down menu with "maximum" and "minimum." (In this case you are obviously looking for a maximum.) At the bottom of the screen you will see a prompt asking you for a lower bound. Type in a lower bound for the search. (When in doubt, type in -10000 or something.) Hit enter. It's now looking for an upper bound. Type an upper bound for the search. (From the graph you can see that the maximum is near 0. I used a lower bound of -10 and an upper bound of 10.)
The calculator will pause for a moment, then give you the xy coordinates of the maximum at the bottom of the screen.
Check your user's manual if any of this doesn't work.
-Dan