Slope: -7
Y intercept: 42
Find the slope with this formula:
y2-y1/x2-x1
So if we plug in the numbers:
7-21/5-3 = -7
-7 is our slope
Now plug in any of the two points into y = mx + b, with m being our slope and solve for b, our y intercept
21 = (-7)(3) + b
b = 42