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The unstated assumption in the question is that the ordered pair is written as (x, y) and not (y, x).
\n" ); document.write( "With this assumption we can then see that by giving the ordered pair (_, 78), the author of the question wants us to understand that y = 78 and x is not known.
\n" ); document.write( "However, x can be found from the fact that y = 78 and the fact that the two variables are related through the equation:
\n" ); document.write( "y = -10x + 18.
\n" ); document.write( "Substituting y = 78 into the equation we can find the value of x.
\n" ); document.write( "78 = -10x + 18
\n" ); document.write( "Take away 18 from both sides of the equation (the purpose is to isolate x on one side).
\n" ); document.write( "78 - 18 = -10x + 18 - 18
\n" ); document.write( "Simplify.
\n" ); document.write( "60 = -10x,
\n" ); document.write( "which is the same as
\n" ); document.write( "-10x = 60
\n" ); document.write( "Divide the whole equation by -10 (the purpose is still the same: isolate a single x to be alone on one side of the equation)
\n" ); document.write( "-10x/(-10) = 60/(-10)
\n" ); document.write( "Simplify
\n" ); document.write( "x = -6
\n" ); document.write( "Solved.
\n" ); document.write( "The ordered pair is (-6, 78) \n" ); document.write( "