Along what types of tectonic plate boundaries
are volcanoes generally found? Why?

Divergent plate boundaries because the separating of the tectonic plates allows for the lava to push its way through the rift and escape to Earth's surface.

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)