Approach this using the Pythagorean theorem.
The height of the line is 8 units and the width of the line is 7 units. The length is the hypotenuse:
L^2 = W^2 + H^2
L^2 = 7^2 + 8^2
L^2 = 49 + 64
L^2 = 113
L = 10.6
The length of segment SE is 10.6 units.