equivalence of two linear PDE

Hello,

i try to understand some basic theory about linear PDE with constant coefficients.

But my knowledge about PDEs and the notation used in books is not so good.(because i have just started to study this topic).

i don't understand this statement or perhaps just the notation :

"for any , solving on is equivalent to solving , where and ."

Ok i don't understand, why this is true. I think the author mean by P(D) some general linear PDE with constant coefficients, that is with constant coefficients.

So but what i don't know how to understand P(D+a)?

And why are both PDEs equivalent? Do you have an idea?

Regards