X2 then add on the value you are multiplying by.

OR 

x4 then subtract the value you are multiplying by.

Pattern Spotting

3,   6,    9,      12,     15,     18 ,   21,    24,    27,    30 ,    33,   36 .   What do you notice?

3 , 6 ,  9   is  the pattern to notice.

add together the digits of 12  (1 +2 =3)

add together the digits of 15 (1 +5 = 6)

add together the digits of 18  (1+8= 9)

and it repeats....  

Use your previous times table knowledge!

Use your 2 times table knowledge to double and double again!

If you know your 3 times table then multiply by 3 then add the number you are multiplying by on to it aswell. 

If you know your 5 times table,  multiply by 5 then subtract the number you are multiplying by.