To get the variable by its self you have to ether add or subtract. Example:
4=7+x+6x
4-7=7x
-3=7x
-3/7=x
Here's another example:
2t-12-3t=60
-t=60+12
-t=72
t=-72 Hope it helps!
Digits range from 0 to 9 That makes 10 double digits
number of permutation of 2 digits when they are different = 10*9 = 90
this gives 90 + 10 = 100 different ways to arrange the digits
combine this with e 26 letters in alphabet we get number different IDs
= 26*100 = 2600