Answer
$-2.1x^2-3.2x-1.7$
Work Step by Step
Removing the parenthesis and then combining like terms, the given expression, $
(1.8x^2-6.8x-1.7)-(3.9x^2-3.6x)
,$ is equivalent to
\begin{array}{l}\require{cancel}
1.8x^2-6.8x-1.7-3.9x^2+3.6x
\\=
(1.8x^2-3.9x^2)+(-6.8x+3.6x)-1.7
\\=
-2.1x^2-3.2x-1.7
.\end{array}
Let
\begin{array}{l}\require{cancel}Y_1=
(1.8x^2-6.8x-1.7)-(3.9x^2-3.6x)
,\\Y_2=
-2.1x^2-3.2x-1.7
.\end{array}
Using a graphing calculator, the graphs of $Y_1$ (dotted graph) and $Y_2$ (solid graph) are shown below. Since the graphs overlap, then $Y_2$ is the correct simplification of $Y_1.$
