voltage\current sources
Pretend, for a moment, that there is a resistor between the positive terminals  of the 10v and 5v supplies. Then the voltage across the resistor is 5 volts. So the current through the resistor is $$i = \frac{5}{R} amps$$ Since the two supplies are connected by a short circuit, with zero resistance, what do you think the current across point a is?