Answer :
The largest x that satisfies
[tex]3x^2+17x+15=5[/tex]
is the largest solution to
[tex]3x^2+17x+10=0[/tex]
We have
[tex]3x^2+17x+10=(3x+2)(x+5)=0[/tex]
[tex]\implies x=-\dfrac23\text{ or }x=-5[/tex]
and so the largest value of x we want is -2/3.