Charge Q is consistently distributed on a line segment that extends along the x-axis of a Cartesian coordinate system from the origin to x=L. Hey this is one of those circumstances in which the linear charge density is the same everywhere on the line segment and the linear charge density is everywhere equal to the average charge density. One is request to find the electric potential valid for points on the positive y-axis. What must one use for the distance r that appears in the equation for V below copied directly from your formula sheet?

V=kq/r

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Let x' be the distance that a particular infinitesimal charge element is from the origin. x' is the variable of integration. x' takes on values from 0 to L. For a specified charge element on the x-axis the distance r is √(x'² + y²). Note that r is a function of the variable of integration.