$$5x-1= { 15 }^{ 2 } - { 11 }^{ 2 }$$
$x=21$
$$5x-1=225-11^{2}$$
$$5x-1=225-121$$
$$5x-1=104$$
$$5x=104+1$$
$$5x=105$$
$$x=\frac{105}{5}$$
$$x=21$$
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