You have baked 23 brownies, and you need x more.
The total number of brownies you will bake is x + 23.
There are 28 students, and you need 2 brownies per student.
You need 2 * 28 = 56 brownies.
The number of brownies you will bake must equal the number of brownies you need. Set the two expressions equal to each other, and that is the equation.
x + 23 = 56