Find total resistance using LTspice
Ignoring the problem pointed out in comments where you didn't correctly capture the schematic,

My question is how can I find voltage Vab = Vx ?

Your b node is connected to ground so its voltage (as reported by SPICE) is always 0.
Therefore what you call \$V_{ab}\$ is what SPICE calls V(a), which is reported in the screen image you provided.