Consider the function

$f(x)=2x^3-6x^2-90x+9on$ the interval $-6,7$

$.$ Find the average slope $of$ the function $on$ this

interval.

Average slope:

$By$ the Mean Value Theorem, $we$ know there

exists $at$ least one $cin$ the open interval $(-6,7)$

such that $f^{\text{'}}(c)is$ equal $to$ this mean slope.

Determine all values $ofc$ that satisfy this

condition. $If$ there $is$ more than one value, enter

them $in$ a comma$-$separated list. $(Round$ your

answer $to$ within three decimal places $if$

necessary.$)$

$c=$