Quadrilateral ABCD is inscribed in circle O. What is ​ m∠B ​ ?Enter your answer in the box.

1. The quadrilateral ABCD is inscribed in circle O, then, the sum of its opposite angles is 180°.

 2. As you can see, m∠A=2x-7 and m∠C=x+4 are opposite angles, then, you have:

 m∠A+m∠C=180°

 3. When you substitute the values of m∠A and m∠C into m∠A+m∠C=180°, you obtain:

 m∠A+m∠C=180°
 (2x-7)+(x+4)=180°

 4. You must clear the "x", as below:

 3x-3=180°
 3x=180°-3
 x=177°/3
 x=59°

 5. Now, you can substitute the value of "x" into m∠B=2x+3:

 m∠B=2x+3
 m∠B=2(59°)+3
 m∠B=118°+3
 m∠B=121°

 What is ​m∠B ​ ? 

 The answer is: m∠B=121°