Solving CMOS logic structures
The PMOS turn on when the voltage is low. So you have a pull up network with two parallel legs. The first leg has two transistors in series, which means that both need to turn on for the output to be pulled high. This is your \$\bar{A}\bar{B}\$ term. The second leg has a single transistor. This is your \$\bar{C}\$ term. Because they are in parallel the pull up network will pull the output high, if at least one of the legs is active. Hence \$F = \bar{A}\bar{B} + \bar{C}\$.