All the numbers after the x's are subscript.
Y=13.20 + 2.10x1 +1.24x2 +0.62x3
where
Y= monthly sales revenue (in $1,000's)
x1= monthly advertising expenditure (in $100s)
x2= (x1)^2
x3= a time trend x3=1 in month 1, x3=2 in month 2 etc.
Suppose that in Month 10 advertising expenditure will be $600. Predict revenue for that month.