Calculating the power supplied by the current and voltage sources

Generally the Power is nothing but the product of current and voltage

You are right !

You just have to calculate the voltage across the current source, and the current in the voltage source, multiply the related current and voltage, and you'll get the answers.

• for the current source : the voltage is 12V (the same of the voltage source, because they are in parallel). The power is 6A * 12V = 72W
• or the voltage source : you have first to calculate the current flowing in the source, which is the difference between the current in the resistor and the current of the current source. I = 4A - 6A = -2A. And the power is : -2A * 12V = -24W. The minus sign tells the voltage source will RECEIVE 24W

[ADDED] Oh, and by the way : the power in the resistor is 12V * 4A = 48W. You can check that it is equal to the algebraic sum of the powers of the two sources: 48W = 72W + (-24W)