The y-intercept is the "b" in the slope-intercept form of y=mx+b. Based on the equation you wrote, you can substitute each of your known values into the slope-intercept form.
A geologist in South America discovers a bird skeleton that contains 80% of its original amount of C-14
We use formula 
N0= initial amont of C-14 (at time t=0)
N= amount of C-14 at time t
t= time, in years
k= 0.0001
N is 80% of N0 that is 


Divide both sides by N_o

Take ln on both sides

ln(0.8) = -0.0001t ln(e)
ln(0.8) = -0.0001t
Divide both sides by -0.001
t = 2231.435513
The age of the bird skeleton is 2231 years old
Answer:
3 is a function.
1 and 2 are not.
Step-by-step explanation:
1 and 2 are not functions because we can draw a vertical line which passes through more than one point on the graphs.
You cant do this on graph 3 so it is a function.