It is useful to have names for matrices. Usually a bold lower case letter is used for a column or a row matrix:
a = ( 1.2, -3.6 ) x = ( x1, x2, x3, x4 ) r = ( r0, r1 )T
It is conventional to use names from the start of the alphabet for column matrices whose elements we know (like a above), and to use names from the end of the alphabet for column matrices whose elements are variables.
Often the names of column matrix elements are subscripted versions of the name of the whole column matrix (like column matrix r and its elements r0 and r1).
Bold face is hard to do with pencil or chalk, so instead an arrow or a bar is placed over the name:
_ -> x x
Say that you know:
x = ( x1, x2 ) y = ( 3.2, -8.6 ) x = y
What must be true about x1 and x2?