Step by step
The question
Question: find f(5) for f(x) = 3x − 7.
f(x) is read "f of x".
It is a rule, called a function.
A number goes in for x.
The rule gives one answer out.
There are four choices.
First we solve it, the normal way.
Then we pick.
1 Put the number in
f(5) means: put 5 in place of every x.
We write it in brackets: 3x becomes 3 times 5, not 35.
2 Times and divide
Everyone uses the same order, so we all get the same answer.
Times comes before minus.
3(5) means 3 times 5.
3 times 5 is 15.
3 Plus and minus
15 minus 7 is 8.
So f(5) = 8.
4 Pick the choice
Our answer is f(5) = 8.
That is choice C.
5 Why not the others
A is −4.
Wrong: 3 − 7: left out the 5.
B is 1.
Wrong: 3 + 5 − 7: added 3 and 5.
D is 22.
Wrong: 15 + 7: added 7 instead of taking it away.
Let's look back.
First: put the number in.
Then: times and divide.
Then: plus and minus.
Then: pick the choice.
Last: why not the others.