How did i fail on zs in am2?

[Julie.]

I thought perhaps he had put the wrong values in. He's said he's put in r1 and r2, which would give an incorrect figure.
In fact, there can be quite a bit of confusion when initially learning about r1 and r2 as opposed to R1+R2. This may be where he went wrong.

Ahh I see, yeah that was one of the options I had suggested - end to end rather than calculated R1+R2.

Very well could have been, sorry I thought you just meant using lower case letters in error.