Question 971394
a, altitude
b, base
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{{{system(a=b-3, (1/2)b*a=90)}}}


{{{ba=180}}}
{{{b(b-3)=180}}}
{{{b^2-3b-180=0}}}


Going directly to general solution of quadratic equation:
{{{b=(3+- sqrt(9+4*180))/2}}}
{{{b=(-3+- 27)/2}}}
{{{b=24/2}}}, from the PLUS form
{{{highlight(b=12)}}}
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{{{highlight(a=9)}}}