Equivalent resistance of circuit
That circuit can be simplified only to a point by using only series/parallel transformations. By using those you end up with this circuit:



where \$R12 = R1 + R2 \$, \$R34 = R3 + R4\$ and \$Rs = R8 + R9 + R10\$.

As you can see no further reduction can be done with series or parallel transformations.

At this point you have two possibilities:


Apply a voltage source to the terminals, calculate (using mesh analysis, for example) the current drawn by the circuit and calculate the equivalent resistance as V/I.
Simplify the circuit further applying the so called star/delta transformations.


With the second approach you need in particular these formulas to transform, for example, the delta connection of R5, R6 and R7 into a star connection. Once you do this, the circuit can be further simplified using usual series/parallel transformations.