How do i solve for the missing length? i’m struggling

Let call it x 
x/4=4/2 
->x=8

For the first one: i don't know...i think it can't be done.
for the second one, it is : 4 × (2 + 3) - 1 = 19
for the third one, it is 4 + ((3² +1) ÷ 5) = 6
for the fourth one, it is : (3 + 2) × 6 - 3 = 27
for the fifth one, it is : 7 + 4 - (9 ÷ 3) = 8
for the sixth one, it is : (6 ÷ 2) + (4 × 2) = 11
for the seventh one, it is : (3 + 1)² ÷ 4 = 4
for the eight one, it is : (12 + 20) ÷ 4² = 2
for the ninth one, it is :  7 + (7 - 18 ÷ 6)² = 23