exam Maths Puzzle Numerical Ability

I would appreciate help with this problem. Thank you!
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exam Other Question

Using the formula I=PRT to find simple interest on an account, find the ammount earned in 3 years if you deposited $500 into a saving account earning 6% per year. in this equation i need to use substitution!!
please help me!!!
thank you!

y=3x
x=2y=-21