can someone answer these?

Short Answer
3: C
4: D
Problem Three
Remark
Somewhere we ought to be told that this is the Doppler Effect. I have never done a problem using this formula, so I think I'm doing it correctly, but no guarantees. My guess is that the frequency increases as it comes towards you and decreases as it moves away from you. I think that is correct.
Formula
Givens
f' = (v + vo) * f / (v - vs)
Solution
f'/f = 1.1
1.1 = (3*10^8 + 0 ) / (3*10^8 - vs)
3.3*10^8 - 1.1*vs = 3*10^8
3.3*10^8 - 3*10^8= 1.1 vs
0.3 * 10^8 = 1.1 vs
2.73 * 10^7 = vs
The closest answer is 3.00 * 10^7 which is C
Problem Four
Here what is happening is that you are looking for the frequency resulting from a wave moving towards you at 1/2 the speed of sound. You are not moving.
Givens
f' = v/(v - 1/2v) * 1000
f' = v/ (1/2 v) * 1000
f' = 2 * 1000
f' = 2000 which is D