Answer
$-61$
Work Step by Step
1. For -15, find its 8-bit representation as $(2^8-15)_{10}=(241)_{10}=(11110001)_2$
2. For -46, find its 8-bit representation as $(2^8-46)_{10}=(210)_{10}=(11010010)_2$
3. Add up the above to get $111000011_2$
4. Truncate the leading 1 and treat the result as negative to get
$111000011_2\rightarrow -(2^8-195)_{10}=-61$
