Answer:
upstream 4 km/h
Step-by-step explanation:
Let's define "upstream" as positive direction for the velocities. Therefore, we have:
is the velocity of Danny paddling, which is positive because it is upstream
is the velocity of the current, which is negative because it is downstream
Velocity is a vector, so we have to calculate the resultant velocity. However, the two velocities are along the same direction (just with opposite sign), so their resultant is given by their sum:

and the positive sign means the direction is upward.