Turn off all 20 amp circuit breakers at the circuit breaker box in the basement

If I interpret correctly:

A) Wire (Cable) #2 is yellow. The white wire strand of cable #2 is capped with a red wire nut at 7:30; Remove this wirenut and attach this white strand of yellow cable #2 to Terminal B4. 

B) The red wire nut at 11 o'clock covers only one black wire from one of the white (#14-2) Romex cables.  

C) The yellow wire nut at 1:30 o'clock covers only one black wire from one of the white (#14-2) Romex cables.

D) If B) and C) are correct, then connect the black wire from B) by inserting it with the other black wire already there into the yellow wirenut at 11:30.  ==> Since one of these black wires will be"hot", be careful not to touch the bare tips of either of these wires to anything but to each other and to the inside of the wire nut with which you secure them.