Designing a combinational circuit that converts a 5 bit binary into its 2's complement
As @oldfart suggested, instead of solving all those lengthy K-maps, you can directly draw the combinational ckt by using the relation that:

Two's compliment = One's compliment + 1

if input = \$A_4A_3A_2A_1A_0\$ and output = \$Z_5Z_4Z_3Z_2Z_1Z_0\$ , 
then:

\$Z_5Z_4Z_3Z_2Z_1Z_0\$ = \$A_4^1A_3^1A_2^1A_1^1A_0^1\$ + 00001

You can expand the Circuit further by substituting the equivalent gate level circuit of adders.