So we have the numbers of one triangle and one number from another triangle
(congruent triangles have congruent corresponding sides)
do cross multiplication
Altitude over altitude = side over side
SH/FJ = ST/FG
to substitute
5/7 = 6/x
cross multiplication
(5) (x) = (7)(6)
5x = 42
divide both sides by 5 to isolate the x
x = 8.4
That means that side FG is 8.4
Hope this helps ;)