The numeric expression 3-(2+5) can be simplified by using the Order of Operations rules (Please Excuse My Dear Aunt Sally).  

First you must simplify inside the parentheses -->    (2+5)=7

We are left with the single operation 3-7-->    3-7 = -4

This simplified expression is -4.

The Algebraic Expression 3x-(2x+5) must be simplified differently.  You can't simplify inside the parentheses because 2x and the constant 5 are not like terms.

Using the Distributive Property we can simplify this expression to 3x-2x-5.

We can now combine the like terms to get x-5

This simplified expression is x-5

When you simplify 3-(2+5) all the terms are constants so the result is a constant.

3-2-5=-4

Thus the result is -4.

When you simplify 3x-(2x+5), you need to group the x terms or variable terms and the constants separately.

3x-2x-5=x-5

Thus the simplest form of this expression is x-5.