Not sure what youu mean by "solve this equation" - it's an inequality which is in fact true for the case 'a' and 'x' are both positive and x<a. It's not too hard to see - consider how the area under the curve 1/t from a-x to a+x can be approximated by either (1) the area of a rectangle centerd on t= a, with width 2x and height 1/a, or (2) the area of the rectangle using height equal to the average of the values for t= a-x and t=a+x, and width of 2x.