The height (in meters) of a shot cannonball follows a trajectory given by $h(t) = -4.9t^2 + 14t - 0.4$ at time $t$ (in seconds). As an improper fraction, for how long is the cannonball above a height of $6$ meters?
The cannonball is above $6$ meters in height when $-4.9t^2 + 14t - 0.4 \ge 6.$ Rearranging and multiplying through by $-10$, it follows that $$49t^2 - 140t + 64 \le 0.$$The quadratic expression factors as $$(7t - 4)(7t - 16) \le 0;$$then $7t-4, 7t-16$ have opposite signs, so it follows that $\frac 47 \le t \le \frac {16}7$. The cannonball then spends $\frac {16}7 - \frac 47 = \boxed{\frac{12}{7}}$ seconds above the height of $6$ meters.

[asy]
import graph; size(8.945cm); real lsf=0.5; pen dps=linewidth(0.7)+fontsize(10); defaultpen(dps); pen ds=black; real xmin=-2.935,xmax=7.01,ymin=-3.295,ymax=11.24;

real f1(real x){return -4.9*x^2+14*x-0.4;}
filldraw(graph(f1,-2.925,7)--cycle,rgb(0.95,0.6,0.55),linewidth(1.6));

Label laxis; laxis.p=fontsize(10);

xaxis(xmin,xmax,defaultpen+black,Ticks(laxis,Step=2.0,Size=2,NoZero),Arrows(6),above=true);
yaxis(ymin,ymax,defaultpen+black,Ticks(laxis,Step=2.0,Size=2,NoZero),Arrows(6),above=true); draw((xmin,0*xmin+6)--(xmax,0*xmax+6),linewidth(1.2)+linetype("4 4"));

dot((0.5714,6),ds); label("$A$",(0.755,6.29),NE*lsf); dot((2.2857,6),ds); label("$B$",(2.465,6.29),NE*lsf);

clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle);

[/asy]