This question is asking what the value of g(5) is answer and explain this cause don’t understand functions at

Short g(x) = -3x This is a teaching question. 
Remark
Define and describe f(x)
f(x) is read as y is dependent of x. The notation of f(x) with some exceptions is the same thing as y. What it stands for and what it can be used for is to tell you want value x can have if you want a  specific value for x. 

Suppose you are given 
y = x + 2

Then suppose there is another statement above it
Let x = 5
y = x + 2
y = 5 + 2
y = 7

Instead of writing the top statement
Let x = 5
You use the shorthand
f(5) = x + 2
f(5) = 5 + 2
f(5) = 7.

So what does all this come down to. It is just a short way of writing Let x = ?
f(?) is what x will equal. x will become ?. 

Your Question.
Remember that f(x) tells you what x becomes
f(5) means that x = 5.
Is that more than 4 or is that less than 4? It's a serious question. 
Of course 5 is greater than 4. So that means that g(x) = - 3x. You don't have to do anything. Just read it.

Add up all the sides