What is the slope and intercept of y = -x + 6 ?

Slope of $y = - x + 6$ is $- 1$ and intercept on $y$-axis is $6$.
In slope-intercept form of equation of a line $y = m x + c$, $m$ is the slope of the line and $c$ is its intercept on $y$-axis.
The given equation $y = - x + 6$ can be written as
y=(-1)×x+6, hence its slope is $- 1$ and intercept on $y$-axis is $6$.