Write an expression that can be used to multiply 6 times 198 mentally

I would half 198, get

99 and add a 0, this is how you multiply by 5 in your head

198*5 = 990

990 + 200 = 1190 - 2

= 1188

Alternatively we can round 198 to 200: then multiply by 6

200*6 = 1200

Then We subtract the 6 "2's" that we added

1200 - 12 = 1188

The expression for this is:

(6*200) - (6 * 2)