By the end of February, Rachel's family had used 7, 800 kilowatt hours (kWh) of electricity since they had moved in. On average, her family uses 900 kWh of electricity every month. Let a represent the number of months since the end of February and let y represent the electric meter reading, in kWh.

Determine a function using a and y that models the relationship above.

What is the coordinate pair that represents the data for the electricity usage in Rachel's home at the end of June?