Can you please help solve the problem and show what calculations were used to help me better understand the equation?

Thank you

Using the following information regarding actual sales for Sam's Ski Supplies, project sales for March of Year 3 using simple linear regression: Sales for Sam's Ski Supplies ($000s) Month First Year Second Year January 380 400 February 340 360 March 320 330 April 280 290 May 265 270 June 230 235 July 220 230 August 200 205 September 210 220 October 250 270 November 400 450 December 450 502