Question 915744
Try to look for two binomial factors.  First, factorize {{{-1}}} to make the task easier.


Next, can you find a pair of numbers whose sum is 14  (did you forget to include factor of r? )  and whose product is 45.


{{{-1(r^2-14r+45)}}}


3*15=45
5*9=45
Anything else?
No.



The question you really wanted was, what method needed to factor {{{m^2+3m-20}}}.
Not factorable.  The method is the same, just that no need to factor any {{{-1}}}.   What two integers have product of {{{-20}}} and sum of {{{3}}}?  What are they?  They MUST be integers.


Let's check what we can find.
Factorization of 20, ignoring sign:
1*20;
2*10;
4*5.
No others.
Any sum or difference to give positive 3?
-
-
NO.
Not factorable.  PRIME.