Im doing more work on this. I have no answers so id like to see if someone can go over my mine.

1) How many binary strings length 7 begin with 2 0's or end with 3 1's.

2^5 + 2^4.

2)

x1 x2 x3 are not negative numbers.

we have: x1 + x2 + x3 = 11

How many solutions would there be for the equation?

ok so im thinking this:

11! * 3.

3) let x1 <equal to 3, x2 <equal to 4 and x3 <equal to 6.

Im thinking if my idea of answering question 2 is correct, then the same would be for this one.

3! + 4! + 6!

Any confirmation from math helpers appreciated.