$.(10$ points$)$ Archer Company is a wholesaler of custom$-$built air$-$conditioning units for commercial buildings. It gathered the following monthly data relating to units shipped and total shipping expense:

Month Units Shipped Total Shipping Expense

January $30$ $ $1,800$

February $60$ $ $2,300$

March $40$ $ $1,700$

April $50$ $ $2,000$

May $70$ $ $2,200$

June $90$ $ $2,910$

July $20$ $ $1,300$

$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$

Required: Using the high$-$low method, calculate the equation of a cost line for the shipping expense for this company $($Y $=$ a $+$ bX$)$