$2\mathrm{.}10.$ Section $2\mathrm{.}4$ gives two algorithms for solving triangular systems. One subtracts columns of the triangular matrix from the right$-$hand side; the other uses inner products between the rows of the triangular matrix and the emerging solution.

$($a$)$ Which of these two algorithms does bslasht $x$ use?

$($b$)$ Write another function, bslashtx$2,$ that uses the other algorithm.