How do I format negative fractions so that the minus sign is outside the fraction?

[DuncanMarshall]

I’m trying to format a fraction so that the minus sign is outside and to the left of the fraction, using the formula editor. In the included screenshot, I’ve achieved this by not using the formula editor, but that’s ugly for various reasons, so I’d like to do it formula editor. Everything I try in formula editor results in the lower form, where the minus sign pops in to the numerator.

Any ideas?

[PKG]

- {{ 8ab } over { 12a^3 }}

[DuncanMarshall]

@PKG

• {{ 8ab } over { 12a^3 }}

Excellent, thanks.