Solve for r
r=\frac{s+K}{3}
Solve for K
K=3r-s
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3r-s=K
Swap sides so that all variable terms are on the left hand side.
3r=K+s
Add s to both sides.
3r=s+K
The equation is in standard form.
\frac{3r}{3}=\frac{s+K}{3}
Divide both sides by 3.
r=\frac{s+K}{3}
Dividing by 3 undoes the multiplication by 3.
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