In the diagram shown, lines QR and TS are parallel to each other. Also along the line QR, angles on a straight line equals 180 degrees, therefore the unlabelled angle equals;
Q + TQR = 180
Q + 105 = 180
Subtract 105 from both sides of the equation
Q = 75
Upon close observation, you would see that line QT is a transversal of the two parallel lines, hence angle Q and angle 3x are alternate angles. Alternate angles are equal, therefore
3x = 75
Divide both sides of the equation by 3
x = 25
