The equation would be 63500-700x=P, where 63500 is the original population, -700 is the amount the population decreases each year, x is the number of years, and P is the total population.
To answer the second part of the question, plug in 53000 for P and solve for x:
63500-700x=53000
*Subtract 63500 from both sides*
-700x=-10500
*Divide both sides by -700*
x=15
It would take 15 years for the population to drop to 53000.
Hope this helps!!