Because all real numbers can be expressed as fractions, whole numbers can be too, with the denominator as 1.
e.g. 3 = 3/1
And then it works like normal multiplication
3/1 x 3/4 = (3 x 3) / (1 x 4)
= 9/4
= 2 and 1/4
Also for future reference, normally * (the asterix, shift + 8) is used to show multiplication on a computer, because "x" is normally used in algebra to mean something else.
