To solve the problem, we need to understand that the total capacity of the shelf is 58 cans, and Alexei can still add 39 cans before it is full.
Let \( c \) be the number of cans already on the shelf. The equation representing the situation is:
\[ c + 39 = 58 \]
To isolate \( c \), we can subtract 39 from both sides of the equation:
\[ c = 58 - 39 \]
Now, calculating:
\[ c = 19 \]
So, the number of cans already on the shelf is 19.
The correct equation is:
c + 39 = 58
And the answer is that there are 19 cans already on the shelf.