Two lense of power P1 = 2 diopters and P2 = 0.5 diopters are placed in contact...

What is the power of the combined lens?

The power is 1/f. where f is the focal length (measured in diopters or 1/meters). if f is in meters , the power of lenses placed in contact simply add , so the combined lens has a power of :

P1+P2 = 2.5 diopters.

When two lenses are placed in contact then the total power of the combination is the sum of their individual powers.
P1 = 2 D.
P2 = 0.5 D.
P = P1 + P2 = 2 + 0.5 = 2.5 D.