Suppose $\sin D = 0.7$ in the diagram below. What is $DE$? [asy]
pair D,E,F;
F = (0,0);
D = (sqrt(51),7);
E = (0,7);
draw(D--E--F--D);
draw(rightanglemark(D,E,F,15));
label("$D$",D,NE);
label("$E$",E,NW);
label("$F$",F,SW);
label("$7$",(E+F)/2,W);
[/asy]
The triangle is a right triangle, so $\sin D = \frac{EF}{DF}$. Then we have that $\sin D = 0.7 = \frac{7}{DF}$, so $DF = 10$.

Using the Pythagorean Theorem, we find that the length of $DE$ is $\sqrt{DF^2 - EF^2},$ or $\sqrt{100 - 49} = \sqrt{51}$.