You are given that all three sides are equal. You also know that all three angles are also equal. Therefore since all three angles are equal and every triangle has 180 degrees, each angle is 180/3 = 60
So
7x - 3 = 60 Add three to both sides
7x - 3 + 3 = 60 + 3 Combine
7x = 63 Divide by 7
7x/7 = 63/7
x = 9 And that's your answer.