Answer
$0.05y^2+1.01y-0.84$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the FOIL Method, the product of the binomials in the given expression, $
(0.1y+2.1)(0.5y-0.4)
,$ is
\begin{array}{l}\require{cancel}
0.1y(0.5y)+0.1y(-0.4)+2.1(0.5y)+2.1(-0.4)
\\\\=
0.05y^2-0.04y+1.05y-0.84
\\\\=
0.05y^2+1.01y-0.84
.\end{array}