If two sides of a triangle measure 12 and 7, which  cannot be the perimeter of the triangle

Hello Tovarmackenzie!
We can utilize the Triangle Inequality Theorem, Which states

A + B > C
A + C > B
B + C > A

With that said, we are going look for C.
We know that
12 + 7 > C
So
19 > C
OR
C < 19
So our 3rd leg has to be less than 19.

So if we make C  =18.999 and Sum up all the sides, 12 + 7 + 18.999 = 37.999
this means the max perimeter is 37.999.. so we know  D: 38 cannot be the perimeter.

A

if 12 is the longest and 7 is the shortest then that would mean the addition of another 7 would be 28, no matter the triangle it is impossible for the perimeter to equal 29