How many distinct binary search trees can be formed which contains the integers 1, 2, 3?.

We have that the sum of two numbers  is 9

this can be written as:

x + y  = 9

where x is the larger number.

Now we want to write:

"the difference between one more than the larger number and twice the smaller number"

First, remember that the difference between A and B is:

A - B

Then "the difference between one more than the larger number and twice the smaller number"

is:

"one more than the larger number" = ( x + 1)

"twice the smaller number" = 2*y

the difference between these is:

(x + 1) - 2*y

Now we can simplify:

We know that:

x + y = 9

then:

y = 9 - x

replacing that in the equation:

(x + 1) - 2*y

we would get:

x + 1 - 2*(9 - x)

x + 1  -18 + 2x

(x + 2x) + (1 - 18)

3x - 17

This means that we can write:

"the difference between one more than the larger number and twice the smaller number"

as: 3x - 17