Question 311066
Plug it in and find out.
{{{6x+9y=20}}}
{{{6(2)+9(7)=20}}}
{{{12+63=20}}}
{{{75=20}}}
That's obviously false, hence (2,7) is not a solution. 
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{{{f(x)=4x^(-4x)}}}
{{{f(0)=4(0^(0))=4(1)=4}}} 
{{{f(-1)=4(-1)^(-4(-1))=4(-1)^(4)=4(1)=4}}}
{{{f(2)=4(2)^(-4(2))=4(2)^(-8)=4/256=1/64}}}