Do you agree? The copper displaces the given volume of water.
Now $rho_"Cu"=8.90*g*cm^3$ OR $rho_"Cu"=8.90*g*mL^-1$, i.e. $1*mL-=1*cm^3$
But by definition, $rho_"density"="mass"/"volume"$
And thus $"mass"=rhoxx"volume"=8.90*g*cancel(mL^-1)xx23.4*cancel(mL)$
$=208.3*g$. Do you follow?