$$110 451 600$$ (using $$365.25$$ days as the average number in a year)
$$\text{Explanation}$$
Here's how you do it:
$$3.5\,years \times \dfrac{365.25\,days}{year} \times \dfrac{24\,hour}{day} \times \dfrac{60 \,minutes}{hour} \times \dfrac{60\,seconds}{minute}$$
$$=110451600 Sec$$