This problem has everything, please solve and show work, if using excel please share solver step by step , if using OPL please share the code and steps also. Please show all work!

4. Consider two projects, 1 and 2, to be completed by a team of experts, and they can do only one project at a time. It takes 7 days to complete project 1, whereas the time of project 2 is 4 days. If the project is switched from 1 to 2, then the set-up takes 2 days. On the other hand, if the project is switched from 2 to 1, then the set-up reduces to 1 day. The projects cannot start before Day 5 and both projects must be completed on or before Day 20. Let x1 and x2 be the start date in days for projects 1 and 2. Two projects will not be processed concurrently: If 1 precedes 2, then the start date of 2 must be greater than or equal to start date of 1 plus the working time of 1 and the set-up time. If 2 precedes 1, then the start date of 1 must be greater than or equal to start date of 2 plus the working time of 2 and the set-up time. Use indicator variable(s) representing the order of working of projects and mathematically formulate the constraint(s) that are verbally written above using indicator variable(s) and big M(s). Calculate the minimum value of M that would work. (15 points)