Create in it's own directory (as with all "STEQ 30 MENU" programs). I
like that it puts all the X's and Y's in order. See your user manual
on the Solver function to learn usage.

INTERPOLATION.SOLVR

<<
'X1=X2 - (X3-X1)*(-Y1+Y2)/(Y3-Y1)'

STEQ 30 MENU
>>