OK so 3r^2-16r-7=5
Subtract five from both sides
3r^2-16r-7-5=0
Add like terms
3r^2-16r-12=0
Factorise it, this means putting it into brackets
(3r + 2)(r - 6)
Find out what multiplies to make negative twelve but adds to make negative sixteen
However none of the terms do this so we put negatives next to the "r"s
(-3r + 2)(-1r - 6)
The answer is r=6
Then by similar substitutions, you can easily find that you end up with
Of course, this all assumes that the integrand is continuous over the domain of integration, which would require that are chosen such that for any . If in particular , then we can write