Consider the following $6$ empty spaces: __ __ __ __ __ __
You want to 'fill them up' with letters or numbers. Notice there are $26$ letters in the alphabet and $10$ digits to choose from.
$(1)$ If you want to the password to start with a number, you can pick the first space to be 'filled' with any digit from $0$ to $9,$ so in $10$ possible ways. Since the rest doesn't matter for the condition to be fulfilled, any subsequent space can be filled with a number or a letter of which we have $26+10=36
$ total. So we have $10 \cdot 36^5$ total combinations that start with a number.
$(2)$ If you want the password to end with a letter, for the last space pick any of the $26$ letters available. For the other spaces pick any of the $36$ numbers or letters.So we have $26 \cdot 36^5$ total combinations that end with a letter.
However, notice that there are some combinations that start with a number and end with a letter. If we just sum up $(1)+(2)$ we would be counting these combinations twice (once every time we consider $(1)$ and another time when we consider $(2)$) so we have to subtract a set of these combinations, which is given by $10 \cdot 36^4 \cdot 26$ because of the same arguments as above. Hence the final result will be
$$10 \cdot 36^5\ +\ 26 \cdot 36^5\ -\ 10 \cdot 36^4 \cdot 26.$$