Ideal Voltage and Current Sources in series
The current source tells us that each element in this loop will have a current of 2mA flowing through it, including the resistor. Ohm's law tells us that

$$V_{R} = I_{R}\cdot R_{R} = 2\ mA \cdot R_{R}$$

As you said, KVL gives us that

$$\sum V_{i} = 0V $$
$$\Leftrightarrow -V_{O} + V_{R} + V_{S} = 0V $$
$$\Leftrightarrow -V_{O} + V_{R} + 24V = 0V$$
We can now just insert the first into the second and solve for \$V_{O}\$, and this gives us: 
$$\Leftrightarrow -V_{O} + 2\ mA \cdot R_{R} + 24V = 0V$$
$$\Leftrightarrow V_{O} = 24V + 2\ mA \cdot R_{R}$$