  We discuss an obstruction to a knot being smoothly slice that comes from
minimum-genus bounds on smoothly embedded surfaces in definite 4-manifolds. As
an example, we provide an alternate proof of the fact that the (2,1)-cable of
the figure eight knot is not smoothly slice, as shown by
Dai--Kang--Mallick--Park--Stoffregen in 2022. The main technical input of our
argument consists of gauge-theoretic obstructions to smooth small-genus
surfaces representing certain homology classes in
$\mathbb{CP}^2\#\mathbb{CP}^2$ proved by Bryan in the 1990s.
