Resistor Divider PMOS Example Problem
No, that's right. I'll give you a couple of hints and hope you get them. The voltage \$V_{SG}\$ is equal to \$V_{DD}-V_G\$. Now what's \$V_{G}\$? Apply KCL at the gate and solve for \$V_{G}\$. 

Also sometimes it's worth checking your solution to find out if it makes sense. For example, in terms of your own formula if you put \$V_{DD}=V_x\$, then \$V_{SG}\$ will be equal to \$V_{DD}\$!! But how can that be possible? The voltage across source-gate is equal to the voltage across \$R_1\$. Because the latter is zero the former will also be 0V. So sometimes it's good to check your formulas. 

I hope it helps.