we can calculate the slope of this line passing through (4,5) and (8,9) like this:
m = (y2 - y1)/(x2 - x1)
m = (9 - 5)/(8 - 4)
m = 4/4
m = 1
so the slope is 1, and now we can use the equation of one point and the slope to write the line's equation:
y - y1 = m(x - x1)
y - 5 = 1*(x - 4)
y - 5 = x - 4
y = x + 1
that is the equation of the line, its slope is 1 and the y-intercept is 1