When we deal with a geometric figure, it’s always a good idea to sketch it to help us think. Let the length be $x$ meters. The perimeter is given as ${52\ {\rm m}}$. Adding up the rectangle’s 4 sides gives the perimeter. The equation is: $$\begin{aligned}[t] x+x+9+9 &= 52 \\ 2x+18 &= 52 \\ 2x+18 \mathbf{{}-18} &= 52 \mathbf{{}-18} \\ 2x &= 34 \\ \frac{2x}{2} &= \frac{34}{2} \\ x &= 17 \end{aligned}$$ So the rectangle’s length is ${17\ {\rm m}}$. Don’t forget the unit $\textrm{m}$.