Wheatstone bridge and difference op-amp
Many thanks to @Andy aka for the hint.

Also, it would be useful to draw a common ground for the whole circuit. 

In this case it would be obvious that R4 is in parallel with (R7 + R8). The Thévenin resistance for this branch is 37.5 kOhm and Vb = 4.8 Ohm. Then the voltage between R7 and R8 is 3.84 V. According to ideal op-amp model, the same volatge is between R5 and R7.

Now we can calculate Va: (Vs - Va)/20k = Va/30k + (Va - 3.84)/20k. ==> Va = 6.5 V.