Question 1021922
Use the Pythagorean theorem,
{{{X^2+(X+7)^2=(X+8)^2}}}
{{{X^2+X^2+14X+49=X^2+16X+64}}}
{{{X^2-2X-15=0}}}
{{{(X-5)(X+3)=0}}}
Only the positive solution makes sense here,
{{{X-5=0}}}
{{{X=5}}}