The triangle below has a total perimeter of 450 cm. write an equation and solve it to find the length of the longest side of the triangle.

 So you have 2 sides of a triangle known whose perimeter is 450 and you want to know the length of the third side? 

128 + 150 + x = 450 

278 + x = 450 

x = 450 - 278 

x = 172. Makes a heck of a lot more sense to me than 2.15

This is too long-standing now get out