Consider a simple linear regression with an interaction term:
$Y=b_0 + b_1X +b_2Z+b_3XZ$
where $X$ is continuous and $Z$ is a dummy.
I want to find out whether $X$ has a significant impact on $Y$ when $Z=1$
$b_3$ is negative and significant but $b_1$ is positive and not significant.
The total effect of $X$ on $Y$ when $Z=1$ (i.e. $b_1$ + $b_3$) is negative. How can I determine whether this total effect is significant or not?