To find the maximum power that can be delivered to resistor that is specified
You should write equations to find the \$R_{TH}\$ for everything to the left of the 3k resistor. You will find that \$R_{TH}\$ is a function of the unknown resistor value \$R\$. Now, find the value of \$R\$ that gives you the value of \$R_{TH}\$ that results in the maximum power transfer to the 3k resistor.