Question 980934
{{{x+y=21}}}


{{{y=-x+21}}}


slope, -1
y-intercept, 21



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A linear equation in standard form, but not simplified is given.
Multiply both sides by  {{{1/3}}} simplifies into {{{x+y=21}}}.
If you solve this for y in terms of x, you use steps
{{{x+y-x=21-x}}}
{{{x-x+y=-x+21}}}
{{{0+y=-x+21}}}
{{{highlight_green(y=-x+21)}}}


You must recognize that this is the slope-intercept form of the linear equation, and know what the parts of the equation mean.
y=mx+b is a model format for slope intercept form.
m, means slope, vertical change divided by horizontal change;
b, means vertical axis intercept, the y value for when x is 0.