I strongly urge you to use parentheses here, to remove any ambiguity regarding which quantity is being divided by which quantity. I'm assuming that you meant
(<span>x^2−x−12)/(2x^2)÷(x^2+x−6)/2x^3. If that is correct, great. If not, fix it! Once correct, you can perform the divisions from left to right, beginning with (x^2-x-12)/2x^2.</span>
These are technically expressed as fraction. When you divide a fraction by another, you can replace the second fraction with its reciprocal and you can now multiply the fractions together instead.
Now, just multiply across the top and the bottom. You can first cancel out any terms that are repeated or similar.