How to solve this resistive network using divider rules and ohms law
You are very close but you made a mistake that is keeping you from completion.

R2 is in parallel with R3 to equal R2||R3.

R4 is in parallel with R5 to equal R4||R5.

Next note that R2||R3 is in series with R4||R5 so these add together as:
R2||R3 +  R4||R5 to equal RRight.

Finally RRight is in parallel with R1.

I think you can finish from there. 

Edit
Actually if you are trying to find Vab then use R2||R3 as the top resistor in a voltage divider with R4||R5 across 5V. R1 need not be considered unless you were trying to compute total power consumed from the 5V supply.