EXPLANATION
The compound interest formula is

P= Principal = $10,000
Interest rate = 8.4% = 0.084 in decimal form
n= Number of times interest rate is compounded per unit "t" = 12
t= Time = 30 years
Replacing terms:

Multiplying and dividing numbers:

Adding numbers:

Solving the power:

Multiplying terms:

Sally will have $123,200 in 30 years.