RMS values and Phasors
Based on the problem saying,

$$\dot{U} \text{ and } \dot{I} \text{ are in phase.}$$

It seems clear to me that the dot over the variables indicates phasor quantities.  Their magnitudes are given in rms.
So, your solution will be something like this,
$$\dot{U} = U_{rms}\angle \theta° $$
$$\dot{I} = I_{rms}\angle \theta° $$
or, if you set the θ to be your reference then,
$$\dot{U} = U_{rms}\angle 0° $$
$$\dot{I} = I_{rms}\angle 0° $$