(8x+12)+(3y-9)= 8x+3y+3=78
subtract 3 from both sides
8x+3y=78
subtract 8x from both sides
3y=-8x+78
divide both sides by 3
y=-8/3x+26 or y=-2.666666666x+26
substitute for y
8x+3(-8/3)+26=78
subtract the 26 from both sides
8x+3(-8/3)=52
distribute the 3
8x- 7.99999999998=52
add the -7.999999999998 to bother sides
8x=60
divide both sides by 8
x=7.5
substitute for x
8(7.5)+3y=78
60+3y=78
subtract 60 from both sides
3y=18
divide both sides by 3
y=6
so x=7.5 and y=6
I think you would divide the angle measure by two... Does your question give you options (A,B,C,D)?
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EDIT
Multiply by 2
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When I solved it I got r= -3.38
I hope it’s right
