With Compounding Interest, if we compound a certain amount of times, we use the formula A = P (1 + r/n)^nt.
Where,
A = Amount after t (time)
P = Principal, or starting amount
R = Interest rate, as a decimal (ex. if 6%, .06)
N = Number of times in which the value is compounded per year (semiannually = 2, quarter-annually = 4, etc...)
T = The amount of time the money is being compounded over.
So, because the amount of times your value is compounded is the only thing that changes, we will get a value of A every time. All we have to do to the equation is change the amount of times the value is compounded.
So our equation looks like this: A = 900(1 + .06/n)^n(10)
a) Annually, n = 1, because once per year. A = 900(1 + .06/1)^1(10)
b) Semiannually, n = 2, because twice per year. A = 900(1 + .06/2)^2(10)
c) Quarterly, n = 4, because 4 per year. A = 900(1 + .06/4)^4(10)
Hope this helps!!
Best,
Josh