Adding and Subtracting negative and positive numbers

Having an issue when taking a negative number and minusing a positive number. I assume there is a formula. I have attached a copy of the form. Example... Cas, had a total for the month -$642.55, last week, she was in the positive by 195. the total should be
(-$642.55) - $195 =( -$447.55). Instead it is adding the two (even thought $195 is a positive number) and totaling to (-$837.55). Need a formula so that when starting with a negative number and subtracting a positive number returns a lower negative number. (as shown in illustration)

In the illustration, I manually inputed the number. Would love a formula to handle this for me!

Thanks for any help??

I don't see what specifically you need in the sheet... is the red cells? Are those the expected results? If so, of what numbers summed?

What cell has -$837.55 in it?
FYI, the formula in H12
=SUM(H7-H11) can be simply = H7- H11

H18 does not have a formula in it. If you enter
= H12 - H17, you get -837.55

Not sure what you are doing, if you use this:

=SUM(H12,H17)

it give the -447.55

First you say

I want...

Row 18 to be = to row 12 minus row 17.
Row 19 should be = to row 12 minus row 18
Row 20 Should be = to row 12 minus row 19

implying you want to subtract...

then you say

The issue seems to be when I am starting with a negative number and adding a positive number

implying you want to add.

Subtracting a positive number from a negative number gets you a negative number. Subtracting a negative number from a negative number gets you a negative number or a positive number depending on how large the subtracted negative number is.... it is basic math....remember the number line?

See here:

http://www.mathleague.com/help/posandneg/posandneg.htm

and

http://math.about.com/od/prealgebra/ht/PostiveNeg.htm

If you insist on the results you are wanting, then try absoluting (forcing positive) the first number:

=ABS(E12)-E17 copied across.