Two strings are 15 inches and 7 inches long.
You could draw a diagram that has two rulers (that are longer than 12 inches).
Draw a piece of string that is exactly 15 inches on one ruler.
Draw another piece of string on the other ruler at exactly 7 inches.
Then you can find the total length of the two strings by adding 15 and 7 inches.
15 + 7 = 22
The total length of both the strings is 22 inches. You can draw a ruler and a piece of string on it at 22 inches.