Answer:
[tex]v(3\,s) = 106.6\,\frac{ft}{s}[/tex], [tex]v(15\,s) = -11\,\frac{ft}{s}[/tex]
Step-by-step explanation:
The velocity function of the projectile is obtained by deriving the position function:
[tex]v(t) = -9.8\cdot t + 136[/tex]
Now, the velocities after 3 and 15 seconds are, respectively:
[tex]v(3\,s) = -9.8\cdot (3) + 136[/tex]
[tex]v(3\,s) = 106.6\,\frac{ft}{s}[/tex]
[tex]v(15\,s) = -9.8\cdot (15) + 136[/tex]
[tex]v(15\,s) = -11\,\frac{ft}{s}[/tex]