I'm trying to determine what the perimeter is when the following are given:
5b^2+8b-12
b-4
18b^2-3b+11

Question

I'm trying to determine what the perimeter is when the following are given:
5b^2+8b-12
b-4
18b^2-3b+11

mathstudent55 · Answer

Is it a triangle?

mathstudent55 · Answer

Are the expressions the lengths of the sides?
The perimeter of a polygon is the sum of the measures of the sides.

anonymous · Answer

yes, it is a triangle so the b-4 is the base

anonymous · Answer

I added the equations:
23b^2+5b-5

mathstudent55 · Answer

You need to add the lengths of the three sides:

$(5b^2+8b-12)+ (b-4) +(18b^2-3b+11)$

The parentheses can be dropped because they are unnecessary since you adding the three expressions:

$=5b^2+8b-12+ b-4 +18b^2-3b+11$

You need to combine like terms. Like terms have the same variable parts.

$=5b^2+18b^2+8b+ b-3b-12-4 +11$

$=23b^2+6b-5$

You were pretty close. You just missed the b term of the second side. 
Keep in mind that $b = 1b$, so

$8b+ b-3b$

$ = 8b + 1b - 3b$

$ = 9b - 3b$

$ = 6b$