Linear Interpolation and Extrapolation

I am still new in mathcad and I am wondering if there is a way to get around the fact that linterp(vx,vy,x) cannot use arrays with a different number of elements? For example, three pairs of points but you are missing the last point in one of the pairs; how can you use linear interpolation, rather than a simple algebraic slope proble, to solve it? Also, I know that linterp(vx,vy,x) will extrapolate if x is not within the arrays vx or vy, but how accurate is it? Is there a separate function for extrapolation? Please see attached.

Basically I am looking to replace to two formalas I habe with a short built in function.

Many thanks,
Craig