Solve for r
r=\frac{s+k}{2}
Solve for k
k=2r-s
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2r-s=k
Swap sides so that all variable terms are on the left hand side.
2r=k+s
Add s to both sides.
2r=s+k
The equation is in standard form.
\frac{2r}{2}=\frac{s+k}{2}
Divide both sides by 2.
r=\frac{s+k}{2}
Dividing by 2 undoes the multiplication by 2.
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