Solve for a_13
a_{13}=a_{18}-25
Solve for a_18
a_{18}=a_{13}+25
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-a_{13}=25-a_{18}
Subtract a_{18} from both sides.
\frac{-a_{13}}{-1}=\frac{25-a_{18}}{-1}
Divide both sides by -1.
a_{13}=\frac{25-a_{18}}{-1}
Dividing by -1 undoes the multiplication by -1.
a_{13}=a_{18}-25
Divide 25-a_{18} by -1.
a_{18}=25+a_{13}
Add a_{13} to both sides.
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