step 1:
change the -6c, as a difference
-6c will become +4c -10c (because +4c -10 c is equals to -6c)
step 2:
factor out c from the expression
c² +4c -10c - 40 will become
c (c+4) - 10 (c+4)
step 3:
factor out c+4 from the expression.
c ( c+4 ) -10(c+4)
