regression analysis and time series

the overhead cost for RP Limited have been found to be accurately represented by th fomula
Y=$10000+$0.25X
where Y is the monthly costs and Xrepresents the activity level measured as number of orders.
Monthly activity levels of orders may be estimated using a combined regression analysis and a time series model
a=100000+30b
where a represents the de-seosonalised monthly activity level and b represents the month number.
In month 240 the seasonal index value is 108
REQUIRED
Calculate the overhead costs for RP Limited for month 240 to the nearest $1000