Idon't understand this problem can you ?

Comment
The question is the same as drawing a Black marble. That's one way to look at it. Another is to draw a gray and then subtract that probability of not drawing a gray. I'll do both. Choose the one that is clearest in your mind.

Method One
Total number of marbles = 21 + 11 = 32 marbles of both colors. Probability of drawing a black marble = 
P(B) = P(~gray) = 11 / 32

Method Two
A. Find the probability of drawing a gray marble.
There are 21 gray marbles and 32 total
P(G) = Number of Gray's / total
P(G) = 21 / 32
P(~G) = 1 - 21/32
P(~G) = 32/32 - 21/32
P(~G) = 11/32  <<<< Answer