Answer
$155,480,000.$
Work Step by Step
We know that the number of arrangements of $n$ objects in $r$ slots (where in a slot only $1$ of the $n$ elements can be put) is: $n^r$.
If there are $k$ experiments, the first one can be done in $a_1$ ways, the second one in $a_2$ ways... the last one in $a_k$, then there are $a_1\cdot a_2\cdot...\cdot a_k$ ways of doing the experiments together.
Hence here, since there are $26$ letters for the first two choice, $26-3=23$ for the third and $10$ digits for the $4$ numbers, thus the number of possibilities: $26^2\cdot23\cdot10^4=155,480,000.$
