Question 98450
Use the distance formula 

D = {{{ sqrt ( (x2 -x1)^2 + (y2 -y1)^2 )}}}
10 = {{{ sqrt ( (x -4)^2 + (3 -(-3))^2 )}}}
square on both sides

{{{100 = (x-4)^2 + 36 }}}
{{{ (x-4)^2 = 100 -36}}}
{{{(x-4)^2 =64}}}
x-4 = +-8
x=12 and x=-4 

So your answer of 12 is also correct