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f(-21)=-2(-21)+3
We move all terms to the left:
f(-21)-(-2(-21)+3)=0
We add all the numbers together, and all the variables
f(-21)-45=0
We multiply parentheses
-21f-45=0
We move all terms containing f to the left, all other terms to the right
-21f=45
f=45/-21
f=-2+1/7
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