Answer:
a) 
b) 

Explanation:
Searching the missed information we have:
E: is the energy emitted in the plutonium decay = 8.40x10⁻¹³ J
m(⁴He): is the mass of the helium nucleus = 6.68x10⁻²⁷ kg
m(²³⁵U): is the mass of the helium U-235 nucleus = 3.92x10⁻²⁵ kg
a) We can find the velocities of the two nuclei by conservation of linear momentum and kinetic energy:
Linear momentum:


Since the plutonium nucleus is originally at rest,
:
(1)
Kinetic Energy:

(2)
By entering equation (1) into (2) we have:
Solving the above equation for
we have:

And by entering that value into equation (1):
The minus sign means that the helium-4 nucleus is moving in the opposite direction to the uranium-235 nucleus.
b) Now, the kinetic energy of each nucleus is:
For He-4:

For U-235:

I hope it helps you!
Light waves travel in straight lines when they are travelling in a uniform medium. This is because the waves are travelling at the same speed.
Answer:
because they are found freely in nature uncombined so they are highly reactive with other elements
They correspond due to the fact that certain elements are grouped together based on the number of valence electrons on the outer shell of their electron configuration.
For instance, sulfur (S) has 6 electrons in the outer shell, as it is in the 6th group EXCLUDING transition metals (note, never take these into account, as their charge is always +2; a cation)