Start with the initial population: a = 750 (there are about 750 collared pikas now).
The population decreases by 1/5 each year, so the fraction that remains each year is 1 - 1/5 = 4/5. Thus b = 4/5 (which is 0.8 as a decimal).
Putting these into the form y = a(b)x gives:
y = 750(4/5)x
Equivalently, you can write: y = 750(0.8)x.