$textbf{Concept:}$
The overall momentum of the birds have the same direction as that of goose i.e. in right direction however the goose has momentum larger then that of two ducks. We use the concept of momentum to find the reduced speed for the goose to make overall momentum zero.

$textbf{Solution:}$

The momentum of the goose must be reduced in order to make the overall momentum zero as the speed of goose is larger then combined speed of two ducks.

Setting total momentum zero and solving for speed of goose we get:

$$
vec{P}_{total}=vec{P_d}+vec{P}_d+vec{P}_g
$$

$$
0=-2m_d v_d+m_g v_g
$$

$$
frac{2m_d v_d}{m_g}=v_g=frac{2(4.0,kg)(1.1,m/s)}{7.6,kg}
$$

$$
boxed{bf color{#4257b2}v_g=1.2,m/s}
$$

$$
v_g=1.2,m/s
$$

$textbf{Concept:}$
We need to find the momentum of the two cars and then add them up as the total momentum is the vector sum of the momentum of individual cars.

$textbf{Solution:}$First step is to find the momentum for car

$$
p=m_{car}v_{car}=(1200,kg)(15,m/s)=18000,kg.m/s
$$

Then add the components we get

$$
vec{p}_{total}=vec{p}_{car 1}+vec{p}_{car 2}
$$

$$
{p}_{total,x}={p}_{car 1,x}+{p}_{car 2,x}=0+(18000,kg.m/s)
$$

$$
{p}_{total,y}={p}_{car 1,y}+{p}_{car 2,y}=(18000,kg.m/s)+0
$$

The total momentum is the vector sum of the two components

$$
p_{total}=sqrt{p^2_{car1,x}+p^2_{car1,y}}
$$

$$
p_{total}=sqrt{(18000,kg.m/s)^2+(18000,kg.m/s)^2}=25000,kg.m/s
$$

$$
boxed{bf color{#4257b2}vec{p}_{total}=2.55 times 10^{4} ,kg.m/s,at,45^circ ,north,of,east}
$$

$$
p_{total}=2.55cdot 10^{4},text{kg.m/s},text{at},45^circ ,text{north of east}
$$