The expression to differentiate includes [tex]cos(x)[/tex].
Remember that the derivative of [tex]cos(x)[/tex] is [tex]-sin(x)[/tex].
Put another way, [tex] \dfrac{d}{dx}[cos(x)]=- sin(x) [/tex]
[tex] \dfrac{d}{dx}[cos(x)-7x] [/tex]
[tex]\dfrac{d}{dx}[cos(x)]-7 \dfrac{d}{dx}(x) [/tex]
[tex]-sin(x) -7*1[/tex]
[tex]-sin(x)-7[/tex]
In conclusion, [tex]\dfrac{d}{dx}[\cos(x)-7x]=-\sin(x)-7[/tex]
