Adding up the coefficients of a contemporaneous and a once-lagged variable

#1
Hello,

I'm stuck on what's probably a trivial problem, but I can't figure out how to go about it as I am a semi-beginner, so I would really appreciate the help of anyone who can assist me.

I'm doing empirical work and I want to combine the effects of a variable and a once-lag of the variable by adding up their coefficients from an OLS regression, to put in a table. If I add up the coefficients of a contemporaneous and a once-lagged variable, how can I find the standard error of the resulting coefficient?

Thank you for your time!
 
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