The arrangement is symmetrical, so we can do our calculations on one half of the wall.
(1) The width of the wall is 78.4 cm, so start at the center, at 39.2 cm.
(2) There are to be 2.6 cm between the two pictures, so move left half of that distance from the center to find the right edge of the left picture. 39.2-1.3 = 37.9.
(3) The width of each picture is 12.5 cm. Move left 12.5 cm from 37.9 to find the left edge of the left picture at 25.4 cm.
(4) There are to be 4.5 inches between the two nails for hanging each picture; the width of the picture is 12.5 cm. For the picture to hang straight, you want to have the two nails the same distance from the edges of the picture. So the distance from each nail to the edge of the picture is half of 12.5-4.5 cm, which is 4 cm.
(5) Move right those 4 cm from the left edge of the left picture at 25.4 cm to find the first nail at 25.4+4 = 29.4 cm.
ANSWER: 29.4 cm
A rough diagram (not to scale!)....
0 39.2
|-----------------------------------------|
| 1.3 |
37.9 (R edge of L picture)
| 12.5 |
25.4 (L edge of L picture)
| 4 | 4.5 | 4 |
29.4 (first nail)