Assuming the two horizontal legs are parallel, the smaller triangle is similar to the larger one, so that corresponding sides occur in a fixed ratio. In particular, you have
(6 cm) / (4 cm) = (6 cm + b) / (7 cm)
Solve for b :
(6 cm) • (7 cm) / (4 cm) = 6 cm + b
b = (6 cm) • (7 cm) / (4 cm) - 6 cm
b = 9/2 cm = 4.5 cm