By switching the unknown (x) with the number and seeing if one side really equals the other.
For example, by solving the equation 8/2+3*4+2*X= 10+5*x, we will find that x is the number 2, or that number two is the solution to our equation. To check whether this is true, we have to switch the letter x in the equation with the number 2, so if the results on both sides are the same, our solution is correct.
In this case 8/2+3*4+2*2=10+5*2
Or 4+12+4=10+10
20=20
Which proves that the number 2 is really the solution to the equation.
