The simple interest formula is I=PRT. Plug all of the numbers in.
Interest = 3150
Principal = 5000
Rate = r
Time = 9
3150 = (5000)(r)(9)
Simplify.
3150 = 45000(r)
Divide both sides by 45000.
.07 = r
Move the decimal two places to the right to make it a percentage.
7%