I want o estimate the standard errors for sum of OLS coefficient. What would be the formula?

Let's say I have a model

and I want to estimate the standard errors of (b2 + b3), where b1 is coefficient on x2 and b2 on x1*x2.

How to proceed?

Let's say I have a model

Code:

reg y x1 x2 x1*x2 x3

How to proceed?

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