The small triangle containing both the 6 and the ten should have its third side calculated.
Use a^2 + b^2 = c^2
a = 6
c = 10
6^2 + b^2 = 10^2
36 + b^2 = 100
b^2 = 100 - 36
b^2 = 64
b = 8
Note: - 8 has not meaning.
Now by similar triangles using the small triangle and the middle sized triangle on your right
6/8 = 8/(x - 6) Reduce 6/8. You don't have to but it makes things easier.
3/4 = 8/(x - 6) Cross multiply
3(x - 6) = 4*8 Remove the brackets.
3x - 18 = 32 Add 18 to both sides
3x = 50 Divide by 3
x = 50/3 = 16.667 <<<=== answer