Answer:
Since we are given that the ratio are equivalent of the length and width so we will complete our table as follows:
Length(cm) Width(cm)
- 4
3 -
5 10
7 -
Since we are given 3 blanks and only ratio could be depicted from the third entry.

Hence, all the ratio's must be
.
so from first entry we have:

Where L denotes the length.
for second entry:

where W denotes the width.
for last entry:

Hence the table could be completed as:
Length(cm) Width(cm)
2 4
3 6
5 10
7 14