You would do 4a+3 times 4a-3 which gets you $${\mathtt{16}}{\mathtt{\,\times\,}}{{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{\mathtt{9}}$$ for the fist part!
Then for the secound part $${{\mathtt{4}}}^{{\mathtt{2}}} = {\mathtt{16}}$$, $${{\mathtt{a}}}^{{\mathtt{2}}}$$ is just $${{\mathtt{a}}}^{{\mathtt{2}}}$$ and $${{\mathtt{3}}}^{{\mathtt{2}}} = {\mathtt{9}}$$ so you get the same eqaution as above!
And finally for the last part there is no working out (simplifying) as it's the answer!
All that the equation is doing is showing you how to go from (4a+3)(4a -3) to a more simplified answer.
