Since it is clear that you understand the differences between the LIFO and FIFO methods, let me help you with your other questions, and then you will be able to complete question 2.
The weighted average is the sum of the individual values divided by the sum of the units. So, given the data here, we multiply the costs by the number of units for each of the purchases, and then divide by the total number of units.
[(600·$40) + (360·$37) + (150·$25) + (200·$45) + (580·$42)] divided by 1890 units
The specific identification units sold means that you identify which units were sold from the inventory purchased on a given date. When you look at the units sold on March 15, you will need to identify units came from the prior purchases. That evaluation will depend on the use of LIFO or FIFO policies.