Step-by-step explanation
We can affirm by the triangle midsegment theorem that the segment DF is half of the segment BC, therefore:

Plugging in the terms into the expression:

Applying the distributive property and removing the parentheses:

Subtracting -3x to both sides:

Subtracting terms:

Adding +3 to both sides:

Adding numbers:

In conclusion, the value of x is 6