We'll use the slope-intercept form y = mx + b. Where m is the slope and b is the y-intercept.
We know the slope, -1/5. Now the equation looks like y = (-1/5)m + b
To find the y-intercept, plug in the ordered pair that the question gave us into our equation and solve for b.
7 = (-1/5)(-3) + b
7 = 0.6 + b
6.4 = b
b = 6 2/5
So, the y-intercept will be at 6 2/5.