The nth term of a quadratic sequence is: n²+2n-4 work out the first three terms of this sequence.

We can work out the first three terms of this quadratic equation with substitution.
The first term of this sequence is where n=1, we can substitute 1 in for n
y=1^2+2(1)-4
y=1+2-4
y=-1
We can do this for the next two terms as well
y=2^2+2(2)-4
y=4+4-4
y=4
For the last term:
y=3^2+2(3)-4
y=9+6-4
y=11

No a triangle cannot be formed because when you add the two smallest number together (which would be 11) the have to be bigger than the number it is "against" ( which would be 12)