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84-20a+a^{2}=\left(8-a\right)^{2}
Use the distributive property to multiply 6-a by 14-a and combine like terms.
84-20a+a^{2}=64-16a+a^{2}
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(8-a\right)^{2}.
84-20a+a^{2}+16a=64+a^{2}
Add 16a to both sides.
84-4a+a^{2}=64+a^{2}
Combine -20a and 16a to get -4a.
84-4a+a^{2}-a^{2}=64
Subtract a^{2} from both sides.
84-4a=64
Combine a^{2} and -a^{2} to get 0.
-4a=64-84
Subtract 84 from both sides.
-4a=-20
Subtract 84 from 64 to get -20.
a=\frac{-20}{-4}
Divide both sides by -4.
a=5
Divide -20 by -4 to get 5.