21 plus 12 in Engpsh is "21 + 12". This is a basic arithmetic operation, where the plus sign "+" represents the operation of addition.

The plus sign comes from the Latin word "plus", which means "more". It has been used in mathematics for centuries to indicate that two or more quantities should be added together.

Here are some examples of how you might use "21 + 12" in Engpsh:

"What is 21 + 12?" (你知道21加12等于多少吗?)
"21 + 12 equals 33." (21加12等于33)
"Can you solve this problem: 21 + 12?" (你能解决这道题吗：21加12?)