Hi.
Voltage divider is circuit that consists of two impedances connected in series. In your case base of CE transistor is connected to the node between resistors and it sinks a non-zero current, so these resistors don't form divider (note that if base current were zero, Ra and Rb could be considered as divider).
You can derive expression for Vb in the next way:
1) Apply KCL to the node between Ra and Rb: Ira = Irb + Ib.
2) Apply Ohm's law to Ra and Rb: (Vcc-Vb)/Ra = Vb/Rb + Ib <=> Vb = Vcc*Rb/(Ra+Rb) - Ib*Ra*Rb/(Ra+Rb) => Vb = 2.5 V.