First let's find the value of f(x) when x = -10:
[tex]\begin{gathered} f(x)=3x+5 \\ f(-10)=3\cdot(-10)+5 \\ f(-10)=-30+5 \\ f(-10)=-25 \end{gathered}[/tex]Now, finding the value of f(20):
[tex]\begin{gathered} f(20)=3\cdot20+5 \\ f(20)=60+5 \\ f(20)=65 \end{gathered}[/tex]Finally, finding the value of x for f(x) = 20:
[tex]\begin{gathered} 20=3x+5 \\ 3x=20-5 \\ 3x=15 \\ x=\frac{15}{3} \\ x=5 \end{gathered}[/tex]