How do I calculate the voltage at these locations?
\$V_c\$ is exactly 0 since it is connected to GND which is defined as zero volts.

\$ V_a\$ is exactly 5 volts since it is connected to the positive end of a 5 volt battery whose negative terminal is connected to GND.

\$V_b\$ is at the junction of resistors connected across the battery so its voltage depends on the relative values of the resistors.  The first circuit is a simple series connection.  In accordance with Ohm's law, the current is equal to the voltage (5 volts) divided by the total series resistance \$(R1 + R2)\$.  \$V_b\$ is equal to the voltage across \$R2\$ which is found by multiplying its resistance by the current.  In this case, then, \$V_b\$ is equal to \$5/(R_1 +R_2)\$ times \$R_2\$ or \$5R_2/(R_1 +R_2)\$.

In the second circuit, the calculation is identical except that \$R_3\$ is now in parallel with \$R_2\$. The equivalent resistance of 2 parallel resistors is given by their product divided by their sum or  \$R_2 R_3 / (R_2 + R_3) \$.  \$V_b\$ in this circuit can then by found by substituting this equivalent resistance for \$R_2\$ in the previous equation.