Recall that the slope-intercept form of a linear equation is:    

     y = mx + b ,  where 'm' is the slope of the line and 'b' is the y-intercept.

     y = 3x + 6     ==>     m = 3 ,   b = 6

Thus, the slope of the line is 3 and the y-intercept is 6.

The y-intercept is when the line crosses the y-axis, so when x = 0

y = 3(0) + 6 = 6, so the y-intercept is 6

the slope is 3, as you know y = mx + b, m is the slope