First off, you should modify you loop. Since you already are tracking how many characters have been entered, using 'i', there is no need to check beyond that in your secondary for loops.
Next, you just have to loop through your lists, and remove whatever you use. Like this:
Ideally, you'd want to track the total number of 'operators' and the total number of 'numbers', and decrement the count each time one is used.
for( x = 0; b[x]; x++ )
if( b[x] == '*' )
a[x] *= a[x+1];
for( y=x+1; a[y]; y++ )
Then you repeat the main loop until there are no more operators.
Stacks and Queues anyone?