Answer
See the answer below.
Work Step by Step
Initial concentration: $0.64\ g\div 487\ g/mol\div 0.025\ L=0.053\ M$
Reacted concentration x, Produced monomer 2x
$K=(2x)^2/(0.053-x)\rightarrow 4x^2+Kx-0.053K$
$x=0.0023\ M$
Monomer concentration: $0.0046\ M$
Dimer concentration: $0.051\ M$