Answer
$\hat y = \bar{y}$
The regression line will be equal to the y-intercept and the mean value of the response variable $y$.
Work Step by Step
Equation of least square regression is:
$\hat y = b_{1}x + b_{o}$
If we put x = 0, then:
$\hat y = b_{1}\times 0 + b_{o}$
$\hat y = b_{o}$
And $b_{o}=\bar{y}$ (the y-intercept is the mean value of the response variable $y$).
