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