To understand the problem, we need to determine how many cans (let's denote this number as \( c \)) are already on the shelf. The shelf can hold a total of 58 cans, and since Alexei can still put 39 cans on the shelf, we can express this situation with the equation:
\[ c + 39 = 58 \]
This equation states that the number of cans already on the shelf (\( c \)) plus the number of cans that can still be added (39) equals the total capacity of the shelf (58).
Now we can solve for \( c \):
- Subtract 39 from both sides:
\[ c = 58 - 39 \]
- Calculate:
\[ c = 19 \]
Thus, the number of cans already on the shelf is 19.
The correct equation and answer from the options provided are:
c + 39 = 58 and 19.