Notice that they give you two specific points marked as (0, 7) and (1, 3) which you can use to find the equation of the line.
First step is to find the slope. Which if two points (x1, y1) and (x2, y2) are given, can be calculated via the formula:
slope = (y2 - y1) / (x2, - x1)
in our case:
slope = (3 - 7) / ( 1 - 0) = -4 / 1 = -4
So the slope is "-4"
Now, we use the general "point-slope" form of a line to find the equation of our line, using our found slope ( m = -4) and either one of the points we used above. We pick (0, 7), to make calculations easier:
General form of the point-slope form of a line:
y - y1 = m (x - x1)
in our case, m = -4, and (x1, y1) = (0, 7) therefore:
y - 7 = -4 (x - 0)
y - 7 = -4 x
add 7 to both sides to isolate y:
y = - 4x + 7
This is the slope-intercept equation of our line.