i will use a slot set up, and fill it in with the info given:
__ __ __ __ __ __ __
the first number can't be zero, this means there are 9 options for the first letter
for the 3 letters, there are 26 options for each letter
the last three numbers include 0, this means there are 10 options for each number
__ __ __ __ __ __ __
9 26 26 26 10 10 10
to account for all the options, the options for each slot should be multiplied
9 x 26 x 26 x 26 x 10 x 10 x 10 = 158,184,000 different licence plates possible