if a=3 and b=-2 what is the value of the expression a-2 b-3
IF your expression "a-2 b-3" is meant to be "(a-2)(b-3)" then Bill is correct, and the answer is -5 when a is 3 and b is -2.
However, if you mean"a - 2b -3" then, substituting the values you gave for a=3 and b=-2, that would be "3 - 2(-2) -3" which is the same as "3 +4 -3" and the expression evaluates to 4.
As in English, punctuation is very important in mathematical expressions: anything surrounded by parentheses must be evaluated first, to get a number, before any operators outside the parentheses are considered. Also note that the multiply (or divide) operators are "stronger" and must be evaluated before addition (or subtraction) are considered.