The estimate of how many stars are in a galaxy is 100 thousand million
Sound waves in air (and any fluid medium) are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction that the sound wave moves.
Well this is just my opinion but I think because all the plants were gone they cant stop mud from rolling into it. Also when it rains the trees wont suck up the water, that will make it over flow and release into the land or dirt on the outsides of it. Mixing those two gives mud and it can potentially flow back in the river in largeish masses causing the river to mix with the mud makinging it more muddy. Hope this helps :D
Answer
given,
position of particle
x(t)= A t + B t²
A = -3.5 m/s
B = 3.9 m/s²
t = 3 s
a) x(t)= -3.5 t + 3.9 t²
velocity of the particle is equal to the differentiation of position w.r.t. time.

------(1)
velocity of the particle at t = 3 s
v = -3.5 + 7.8 x 3
v = 19.9 m/s
b) velocity of the particle at origin
time at which particle is at origin
x(t)= -3.5 t + 3.9 t²
0 = t (-3.5 + 3.9 t )
t = 0, 
t = 0 , 0.897 s
speed of the particle at t = 0.897 s
from equation (1)
v = -3.9 + 7.8 t
v = -3.9 + 7.8 x 0.897
v = 3.1 m/s