Question 377684
Use the vertex form,
{{{y=a(x-h)^2+k}}}
{{{y=a(x-20)^2-10}}}
Then when {{{x=0}}}, {{{y=30}}}
{{{30=a(0-20)^2-10}}}
{{{30=400a-10}}}
{{{40=400a}}}
{{{a=1/10}}}
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{{{highlight(y=(1/10)(x-20)^2-10)}}}