Take your cash sales minus your cash disbursements for each month to get your net cash flow for each month. Then add month one and month 2 net cash flow to get your cmulative cash balance, then add month 3, month 4, month 5, and finally month 6 net cash flows to get your total cumulative cash balance.

For example: month one your cash sales are $300 and your disbursements are $275, so your net cash flow is $25 ($300 - 275). Your month 2 cash sales are $150 and your cash disbursements are $300, so your net cash flow for month 2 is -$150. Now your cumulative net cash flow for month one and two is -$125 ($25 + -150). Now just solve for the remaining months to get your invidual months net cash flows, your cumulative cash balance, and your total cumulative cash balance.