Separating 84 seems to mean that we need to determine what 2 numbers can add up to 84.
84 = x+y
Then we need to figure out the two numbers or "parts" which are the variables x and y. The variables are usually lower case letters of some sort but they are really just placeholders for numbers.
If we say that one part is x then we can follow : x is 9 less (x= - 9) than twice the other : 2y because y is the other. combined, the words give us:
x = 2y - 9
We can plug in x into the previous (84 = x + y) so we only need to solve for one variable:
84 = (2y-9) + y
We need to do algebra to find y by adding 9 to both sides of the equation which gives:
93 = 2y + y
Then 93 = 3y : divide 3 on both sides : 31 = y
If we fill in y = 31 into the 84 = x + y ----------> 84 = x + 31, then x = 53
Checking the answer : one part is 31, then the second part is 9 less than twice the other part: 2(31)-9 which is 62 - 9 = 53