Question 838850
Get the equation of the line containing the two points, and use distance formula to find the length AC.  You want next a point (x,f(x)) where f(x) is the linear function for the line you found.  Use distance formula again for {{{(1/4)*D=sqrt(sumOfTwoQuadraticExpressions)}}}.


The argument inside the square root is an expression in only the variable x.


That is the description of the process.  If you cannot follow that, tell me, and I'll five more detail.