The first step we take is recognising that each term in the expression above can be divided by a number. This number is a factor of 6, 9, and -15, that’s why we say factorise. The highest common factor of these numbers is a 3.
Step 1 would be to setup the brackets, since 3 is the highest common factor we put this outside as shown below.
Step 3 we divide each term in the original expression;
6a ÷ 3 = 2a
9b ÷ 3 = 3b
-15c ÷ 3 = -15c
This gives the factorised expression as;
3(2a + 3b - 5c)