Please help asap (dont need help with the last one i already know the answer is 12 cm)

Alright, he you go: 

Using the rule,

a + b > c

Where a, b, and c are any of the three sides of a triangle, we have three inequalities. 

5 + 8 > x 

5 + x > 8

8 + x > 5

The first one is x < 13

The second one is x > 3

The third one is x > -3

So the interval of x is 3 < x < 13

Assuming that the question is asking for the possible integer, we have: 

4, 5, 6, 7, 8, 9, 10, 11, and 12.

For the second one...using this tool : 

https://www.graphpad.com/quickcalcs/linear2/

The line fo best fit has the equation 

Y = 1.040*X + 5.210

The slope is  1.040

Third one...this is ust the graph of y   = -3x^2  shifted down 6 units

The vertex  of    y  = -3x^2  is at (0,0)

So...the vertex of this function is  (0,-6)

So...the y value of the vertx is  -6