x*=a and x=3

Answer 1

x *= a means x = x * a it works with all math operators e.g. x += a

Answer 2

x = 3 sets 'x' to equal 3

x*= a sets 'x' to equal whatever the value of x already was * the value of a

eg
x = 3
a = 5

x*=a
means
x = 3 * 5
so
x = 15

If x didn't already have a value then x*=a would equal 0 regardless of the value of a as 0 * anything is still 0.