You have just gotten the results from a conversion campaign you ran for a membership...

Return on Ad Spend (ROAS) measures gross revenue generated directly for every dollar spent on the advertising campaign:

ROAS = Revenue From Ad Campaign / Cost Of The Ad Campaign

A more accurate way to calculate ROAS when you have recurring revenue is with the following formula:

ROAS = (# of new customers acquired from Campaign x LTV of a New Customer) – Cost of Ad Campaign

In this case, you would need the following info:

# of new customer => 9

LTV of new customers => 6 * $15 = $90

Cost of Ad Campaign => $400 

ROAS = 9 * $90 - $400 =  $410