Unstable atmospheric conditions lead to the formation of lightning and thunder in
a. nimbostratus clouds.
c. altostratus clouds.
b. altocumulus clouds.
d. cumulonimbus clouds.

D) Cumulonimbus clouds

Cumulonimbus clouds is known to be a dense, lowering vertical cloud that usually formed from water vapor that is carried by powerful upward air currents. Furthermore, this type of cloud is observed during a storms and can be referred to as thunderheads. Thus, cumulonimbus cloud usually forms alone in clusters and are capable of producing lightening and other dangerous severe weather such as tornadoes.

i am positive it is c. condensing but i may be wrong

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.

: -)