First let n represent 'a number'.
Then the 'difference of a number and 7' can be written as: n -7
If twice this equals 18 we can express this with the equation: 2(n-7) = 18
Solve the equation for n:
2(n-7) = 18
n - 7 = 9 <-- divide both sides by 2
+7 +7
n = 16 <-- Add 7 on both sides for answer
CHECK by plugging in 16 to the original statement. "Twice the difference of 16 and 7 is 18"
2 (16 -7) = 2 (9) = 18 ! Therefore our solution is correct