For this case, the first thing you should do is factor the expression correctly.
We have then:
f (t) = t ^ 2-13t + 36
We are looking for two numbers that add -13 and multiply den as results 36:
f (t) = (t-9) * (t-4)
Thus, the zeros of the function are:
t1 = 9
t2 = 4
Answer:
t1 = 9
t2 = 4
