I would take a very visual approach, since the formula is already very visual. Off the top of my head, here are some thoughts:
I would encode minus as cold (and plus as hot, but no plus in this equation). The plus minus looks like a cross or old school TV antenna
I would convert the letters to images they resemble (credit here goes to Gary Lanier and his visual alphabet):
b = a leg with a foot, perhaps kicking something in front of it. Could also be a golf club.
4 = butterfly
a = a tadpole
c = a cookie with a bite taken out of it
2 = a guy with a monocle and a pointy nose
The whole thing looks a bit like a house.
So I’d turn it into a visual story using these elements.
It’s a house. On the top floor there’s a deck off the left side of the attic. Beneath is the first floor
On the deck we find a cold person (shivering) kicking the antenna (to try to get better reception I suppose).
In side the attic is another person, his buddy. He’s got a buck tooth (two) sticking out of his mouth (for the exponent 2). He’s kicking a cold (icy) butterfly while a tadpole nibbles at a cookie.
Meanwhile, down on the first floor, a fancy guy with a monocle is sniffing another tadpole.
Kind of crazy, and there’s plenty of room for improvement here. Maybe for the numbers use images derived from number rhymes (2=shoe) or Major system (2=hen) instead of shapes. Then again, I like the idea of it all being visual, so the whole formula can appear in your mind as is.
Just musing, I haven’t tested any of this. Good luck.