The following information for advertising and sales has been established over the past six months:
Month Sales revenue Advertising expenditure
$'000 $'000
1 155 3
2 125 2.5
3 200 6
4 175 5.5
5 150 .5
6 225 6.5
Using the high-low method which of the following is the correct equation for linking advertising and sales from the above data?