Answer:
To convert from decimal to binary, one approach is to repeatedly divide by 2 as integer division and write down the remainders from right to left:
example: convert 26 to binary
26 / 2 = 13, no remainder => write down 0
13 / 2 = 6, remainder 1 => write down 1
6 / 2 = 3, no remainder => write down 0
3 / 2 = 1, remainder 1 => write down 1
1 / 2 = 0, remainder 1 => write down 1
So 11010 is your result.
For 2's complement, you have to consider youre entire word size. Let's say you have 8 bit representations, then 11010 is really 00011010.
To get the 2's complement, you invert the binary represenation and add 1:
00011010 => 11100101
11100101 + 1 = 11100110 (understand binary addition for this)
So 11100110 is the binary representation of -26.
You can do this entire sequence in reverse, i.e. subtract one, invert and then go back to the decimal representation:
11010 in decimal is 1·2⁴ + 1·2³+ 0·2²+ 1·2¹+ 0·2⁰ = 26