Solve for x and y
-2x-y=-9
5x-2y=-9

we use equation 1 first.
-2x -y = -9 

 -y = -9 + 2x

y = 9 -2x

We substitute value of y in equation 2

5x - 2 (9 - 2x) = -9

5x -18 - 4x = -9

5x - 4x = -9 + 18

x = 18 -9

x = 9

We now find the value of y

-2x -y = -9

-2 (9) - y = -9

-18 - y = -9

-y = -9 + 18

y = 9 - 18

y = -9

large crystals result from very slow cooling.

evaporation of water: this would produce an evaporite which is a sedimentary rock. like a salt pan in a desert. not possible to have volcanic rocks originate this way.

crystalization in molten earth: slow cooling over time. (hundreds to millions of years)

crystalization at earth's surface: this results in fast cooling. (a few hours to a few months or years)

crystalization from cooling off hot water: that would be a hydrothermal deposit. while associated with volcanics they are not volcanic rocks themselves and usually occur fairly rapidly.