Since this is a equilateral triangle all angles are equal to each other. All angles in a triangle always add up to 180 degrees.
So you would set up your equation like so 3(7x - 3) = 180 The 3 outside the parenthesis resembles the 3 angles. You equal the equation to 180 since all the angles in a triangle = 180 So distribute the 3 21x - 9 = 180 Add 9 to both sides 21x = 189 Divide 21 on both sides x = 9
