First, consider that if he cost of 6 uniforms was $90, then, the cost per uniform is:
$90/6 = $15
After the bought of the 6 uniforms, the money the coach has is:
$200 - $90 = $110
the cost of 8 uniforms is:
8 x $15 = $120
$120 is more than the coach has to buy the 8 uniforms.
Hence, there is no enough money to buy 8 uniforms.