The amount of a radioactive substance y that remains after t years is given by the equation y=ae^kt, where a is the initial amount present and k is the decay constant for the radioactive substance. if a = 100, y = 50, and k = -0.035, find t

You will need that formula solved for "t"
(see #4 in the attached graphic.)
a - initial amount
y(t) - ending amount
k - decay constant
t - time
ln - natural log

t = ln[y(t) / a] / k
t = ln[50 / 100] / -.035
t = ln (.5) / -.035
t = -.69314718056 / -.035
t = 19.804205159

Source:
http://1728.org/expgrwt2.htm