Slope-intercept form is: y = mx + b
where m is the slope and b is the y-intercept.
Since you're told the slope is 1/8 and the y-intercept is -7, you just plug those into the form:
y = 1/8 x - 7
Note that the "–" in front of the 7 is already part of the answer on paper, so you don't need to add a second one.