How do you simplify this trinomial? or how do you factor it out? -6a^2-25a-25

The expression cannot be simplified any more, however it can be factored. First, ask yourself, can any number or variables can be factored out? Is this a differences of squares or a perfect square? Since you cannot use any of the methods mentioned, you must use the method of decomposition.
1. Find factors of a*c that equal to b; you need to find 2 numbers, where if you multiply them, it will give the number that you'd get when you multiply a and c (-6 and -25) in this case. The sum of the 2 numbers must also equal to the "b" value (-25 in this case).
2. You know that the sum of the 2 numbers you found in step 1 equals to b. So replace "b" with the sum of two numbers you found. (b=-25=-15a + -10a)
3. Factor out the numbers/variables in each pair
4. Use grouping