Ask the master to solve the math problems of Japanese students' exams ~!
The first step of the answer is to turn this formula into a formula about X, that is, take X as an unknown number and derive an equation of how much X is equal to.
The treatment in the root sign is a complete square treatment.
Because a is a definite number, the number in the square is 0, and then determine the value of a.
You might as well solve it yourself in your own way.