To solve for x, we need to use the fact that the opposite angles of a trapezoid are equal.
Since line f is parallel to line g, the upper left angle of the trapezoid (3x) and the lower right angle of the trapezoid must be equal.
Therefore, we can set up the equation:
3x = 6x + 45
To solve for x, we can subtract 6x from both sides of the equation:
-3x = 45
Then, divide both sides by -3 to solve for x:
x = -15
So the value of x is -15.