Gradpoint ? And the answer is water.
Not only astronomers, but all scientists use the technique of rock dating to determine the age of the universe. From the Big Bang Theory, all the celestial bodies were made at the same time. So, the age of the Earth is just the same with the age of the universe. So, they gather rocks deep down in the crust of the Earth and let it undergo radioactive decay. By determining the amount of radioactive material left and half-life, they could compute the age of the Earth.
Newton's second law of motion states that the force is equal to the product of the mass and acceleration. It can mathematically be expressed as,
F = m x a
When this equation is transformed to solve for acceleration, this becomes,
a = F / m
The equation suggests that for an object to move, force has to be exerted.
You are dealing with pulleys?
can be done with addition of the two equations to eliminate T.

+

=

we can cancel m₁ by dividing both sides by it, assuming mass is not zero

a₂ = 6.125 m/s² ( do significant digits if you need to)
D. a train on a straight track