First, we will distribute parentheses like this:
#x(y+z)=xy+xz#
Therefore, #0.75(x+y)=12# is equal to #0.75x+0.75y=12#
Since we know that #y=-2#, we can put it in our equation
#0.75x+0.75y=12 ->#
#0.75x+0.75(color(red)-color(red)2)=12#
Simplified:
#0.75xcolor(red)−1.5=12#
Since we know that #0.75xx-2=-1.5#, we can change the #+# to a #-#.
Then, we multiply both sides by #100#:
#0.75xcolor(red)*color(red)100-1.5color(red)*color(red)100=12color(red)*color(red)100#
which is equals to
#75x-150=1200#
Add #150# to both sides:
#75x-150color(red)+color(red)150=1200color(red)+color(red)150#
Simplify:
#75x=1350#
Then you divide both sides by #75#
#(75x)/color(red)75=1350/color(red)75#
#75x-:75=x#
#1350-:75=18#
Therefore, #x=18#.