Find the length of db with the length indicated

DB should equal CB plus DC.

First write an expression to represent DC.
(x+168)-(3x+134)
x+168-3x-134
-2x+34

Then write an equal for DC plus CB equals DB
-2x+34+71= 153+2x
-2x+105= 153+2x
-4x+105=153
-4x=48
x= -12

Now plug in the value of x to find DB
153+2(-12)
153-24
129

Final 129

(1/5 x 1M) +( 4/5 x 2M )= 1.8 M