List out all the variables that you do know; 
acceleration=-9.8 ms⁻¹ (this remains constant on Earth)
Final velocity=?
Displacement (s)= -2.1 m 
Initial Velocity(u)=2.5 ms⁻¹
v²=u²+2as 
v²=(2.5)²+2(-9.8)(-2.1)
v²=47.41 
v=√47.41 
v=6.88549 ≈ 6.9 ms⁻¹ 
Hope I helped :) 
        
             
        
        
        
The speed of an object is measured by calculating the distance it travels per unit time. 
The formula in Physics for calculating speed is = distance/ time.
Since an object at rest is not covering any distance, the speed is always assumed to be 0 m/s.
        
             
        
        
        
This is called the Phi Phenomenon.
This is an illusion of movement created when two or more adjacent lights blink on and off in quick succession; when two adjacent stationary lights blink on and off in quick succession; we perceive a single light moving back and forth between them. It is an optical illusion of perceiving a series of still images, when viewed in rapid succession, as continuous motion. 
        
             
        
        
        
Answer:
I believe Mercury has the most extreme temperatures in the solar system, ranging from -280?F at night to 800 degrees F during the day for parts of the surface. 
Hope that helps! :)
 
        
                    
             
        
        
        
Answer:
YOUR ANSWER IS GIVEN BELOW:
Explanation:
The abundance of helium in the universe and the redshift of light from the galaxies.