First, find the slope.
slope = rise / run
slope = (y2 - y1) / (x2 - x1)
slope = (-2 - 3) / (7 - 4)
slope = (-5) / (3)
slope = -5/3
.
Now do point slope form:
y - y1 = m(x - x1)
y - 3 = (-5/3)(x - 4)
That is a valid equation, but let's clean it up a bit.
y - 3 = (-5/3)x + 20/3
y = (-5/3)x + 20/3 + 3
y = (-5/3)x + 20/3 + 9/3
y = (-5/3)x + 29/3