Raku multiplication using prompt and say
To tighten your code a bit, you can declare my $a and assign in the same line:
my $a = prompt &quot;enter number: &quot;;
my $b = 34;
my $c;
$c = ($a * $b);
say  &quot;$a multiplied by  $b is $c&quot;;

The above seems to solve your question, however a user might not respond with the proper input. If a user replies to the prompt with Hello you'll get an ungraceful error:
Cannot convert string to number: base-10 number must begin with valid digits or '.' in '⏏Hello' (indicated by ⏏) in block &lt;unit&gt; at Pedr_Ap_Glyn.p6 line 7

You can add a Type declaration to obviate this issue:
my Int $a = prompt &quot;enter number: &quot;;

...which, if an Int is not returned, will error with the message:
Type check failed in assignment to $a; expected Int but got [WHAT RAKU GOT IS HERE] in block &lt;unit&gt; at Pedr_Ap_Glyn.p6 line 5

Maybe a different pre-declared Type is more useful for your case:
my Numeric $a = prompt &quot;enter number: &quot;;

...which will work with non-Integer input (e.g. 3.14159265).
Or you could declare your own Type subset (below, &quot;PosInt&quot;):
subset PosInt of Int where * &gt; 0;
my PosInt $a = prompt &quot;enter number: &quot;;

...which will only accept Ints greater that zero. Above are just a few options that the Raku language offers--and that you may wish to take advantage of.