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10000=20000*0.91^x
We move all terms to the left:
10000-(20000*0.91^x)=0
We get rid of parentheses
-20000*0.91^x+10000=0
Wy multiply elements
0x+10000=0
We add all the numbers together, and all the variables
x+10000=0
We move all terms containing x to the left, all other terms to the right
x=-10000
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