[tex]\displaystyle \\
L_m = \text{Linia mijlocie } = 6~cm \\
B = \text{Baza mare } = necunoscuta \\
b = \text{Baza mica } = 3~cm \\ \\
L_m = \frac{B + b}{2} \\ \\
2\times L_m = B+b \\ \\
B = 2\times L_m - b = 2\times 6 - 3 = = 12-3 = \boxed{9~cm} \\ \\
Verificare: \\ \\
L_m = \frac{B + b}{2}= \frac{9 + 3}{2}= \frac{12}{2}= 6~cm[/tex]