Answer
See solution
Work Step by Step
$proj_{cu}y=\frac{y\cdot (cu)}{(cu)\cdot (cu)}(cu)=\frac{c(y\cdot u)}{c^2(u\cdot u)}(cu)=\frac{c^2(y\cdot u)}{c^2(u\cdot u)}u=\frac{y\cdot u}{u\cdot u}u=proj_uy$
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