Probably not good enough for an exam answer, but 'wet finger' calcs as follows:
Mach1 at -56C is 570kts appx
.84x570=479kts
TAS increases by appx 1kt/degree warmer than ISA
499 - 479 = 20kts
Therefore temp is roughly 20 degrees warmer than isa = -36C
Mmm - not far off Mathy's more accurate version!!
Cheers
mcdhu