I don't get how this expression simplifies to this answer. I'm supposed to distribute the 5, but I have no idea where that +5 in answer comes from.

To simplify this fraction multiply up and down by the conjugate of the denominator
The conjugate to the denominator has the same terms but different middle sign
Then the conjugate of
[tex]3-\sqrt[]{7}\text{ is 3+}\sqrt[]{7}[/tex]So multiply up and down by 3 + root 7
[tex]\frac{5}{3-\sqrt[]{7}}\times\frac{3+\sqrt[]{7}}{3+\sqrt[]{7}}[/tex]Let us multiply the two denominators
[tex](3-\sqrt[]{7})(3+\sqrt[]{7})=(3)^2-(\sqrt[]{7})^2=9-7=2[/tex]Now let us multiply the 2 numerators
[tex]5\times(3+\sqrt[]{7})=5(3)+5(\sqrt[]{7})=15+5\sqrt[]{7}[/tex]The simplest form of the fraction is
[tex]\frac{15+5\sqrt[]{7}}{2}[/tex]