As given by the question
There are given that the sentence: twice the difference of a number
Now
Twice the difference means to multiply the difference by two
According to the given sentence;
It explains the difference being between the unknown number (b) and 2
So, the answer is said to be 8.
Then,
We need to put the difference (b - 2) and multiply it by 2 and equal to 8
So,

Hence, the equation is 2(b - 2) = 8.