I made a quick and dirty script to see if some combination of + and -'s between all the digits of that string would return 42.

my $str = '7c9695cb23a71d851688cc0d7f8b1072';
my @nums = map {hex} split '', $str;
my @ops = qw(+ -);
my @iter = (0) x (length($str) - 1);

MAIN: while (1) {
    my @set = (@ops[@iter], '');
    my $newstr = join '', map {$nums[$_] . $set[$_]} (0..$#nums);
    print "$newstr\n" if 42 == eval $newstr;

    # Increment
    my $i = $#iter;
    while (++$iter[$i] == @ops) {
        $iter[$i--] = 0;
        last MAIN if $i < 0;
    }
}
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After running for an hour though, I decided that I needed to test this by hand:

use List::Util qw(sum);

my $str = '7c9695cb23a71d851688cc0d7f8b1072';

print sum map {hex} split '', $str;
=prints
231
=cut
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Unfortunately, as you can see, the string contains an odd number of odd digits. This means that no combination of + and -'s between all the digits can return an even number. So there is no simple solution that equals 42