I need help with 7 I really don’t understand this

Step 1
Given;
[tex]\begin{gathered} f(x)=-5(2x+1) \\ f(x)=-35 \end{gathered}[/tex]Required;
[tex]\text{find x}[/tex]Step 2
Substitute f(x) and solve for x
[tex]\begin{gathered} -35=-5(2x+1) \\ -35=-10x-5 \\ 10x=-5+35 \\ 10x=30 \\ \frac{10x}{10}=\frac{30}{10} \\ x=3 \end{gathered}[/tex]Hence x=3