Manage Data Models in Look Challenge Lab

[swalkera8]

I am working through the Looker courses on the Google Skills Boost site and the Challenge Lab for the Manage Data Models quest has me stuck. The first part asks you to create a dimension and a measure for profit and total profit. I was able to get the total profit done correctly but it will not validate my answer for profit. Here is what I had for profit. 

dimension: profit {
    label: "Profit"
    description: "Calculates the profit for the items by subtracting cost from retail price."
    type: number
    sql: ${sale_price}-${TABLE}.cost};;
    value_format_name: usd

}

Am I getting the formula wrong? The code validates as accurate.