hi
A = 9x-4(6-3x)
We are going to develop the part on the right side :
A= 9x-4(6-3x)
A= 9x -4*6 -4*-3x
A = 9x -24 +12x
Now we are going to regroup terms by nature. General rule is to start to with the letter with the higher integrer and so on , and we finish by numbers.
So :
A = 9x+12x -24
A = 21x -24
As we now cannot do any more calculus, we have finish.