This exercise practices spacing, arrays, ellipses and functional names. The image reads as follows (using appropriate fonts and symbols, where <diagonal matrix> is a 4x4 array representing a diagonal matrix with horizontal, vertical and diagonal ellipses representing omissions):

The set of linear equations:
aixi = bi <horizontal gap of 2em> "for all" symbol i = 1,...,n
can be written as a matrix equation:
diag(a)x = b
where x = (x1,...,xn)T, b = (b1,...,bn)T and
diag(a) = [<diagonal matrix>]