Express using a positive exponent:

 d^-3

To make a negative exponent positive, either
- move it up (with the variable), if it is in the denominator, 
or
- move it down (with the variable), if it is in the numerator 

d^-3
--- {wrote d^-3 as a fraction, over 1}
1 

 1
--- {moved d^-3 down into denominator, it became d³}
d³ 

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Solve the equation involving exponents.

5²(5r) = 225
25(5r) = 225 {took 5² to be 25}
125r = 225 {multiplied 35 by 5}
r = 1.8 {divided both sides by 125}

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Simplify.

9^-2 

1
-- {to make a negative exponent positive, move it up or down}
9² 

 1
---- {9 squared is 81}
81 

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