Question 570833
Let's take this one part at a time.  

Let X be the number we're looking for.


Take the sum of this number and 3:
X+3
Two times this:
2(X+3)
is 18


So the equation is 
2(X + 3) = 18
2X + 6 = 18

Subtract 6 from each side giving:
2X = 12

Divide both sides by 2 giving:
X = 6


Checking:
The sum of 6 and 3 is 9
Twice that is 9 X 2 = 18  Correct!