经画图可知
y>=0 x>0 a>0
y^2=ax
y=√(ax)
S=∫(0->3) (√(ax)-0)dx
=√a∫(0->3)x^(1/2)dx
=√a*2/3*x^(3/2)|(0->3)
=2√a/3 *3√3
=2√(3a)=6
√(3a)=3
3a=9
a=3