Hello!
So the equation that you want to use is
y = mx + b
Now you can plug in both the slope and your points to find b.
3 = -1/2(2) + b
3 = -1 + b
4 = b
So now that we've found b, we can use the original equation and only plug in the slope and b
y = -1/2x + 4
So to find the y-intercept, we need to plug in 0 for x
y = -1/2(0) + 4
y = 0 + 4
y = 4
Your y-intercept is (0,4)
Hope this helps!