We know that he weeds gardens for 15 hours, and mows lawns for x hours.
If he wants to earn $252 per month, we can express the following
[tex]9\cdot15+9x\ge252[/tex]Now, we solve for x
[tex]\begin{gathered} 135+9x\ge252 \\ 9x\ge252-135 \\ 9x\ge117 \\ x\ge\frac{117}{9} \\ x\ge13 \end{gathered}[/tex]