What is the perimeter of triangle ▲abc with vertices a(-2 9) b(7 -3) c(8 -4)?

You really need to go back to the start of the course and learn the basics. Vertices are labeled with capital letters and the side opposite is given the lower case letter. Coordinates are separated by a comma and a space.

A(-2, 9) B(7, -3) C(8, -4)

The horizontal distance from A to B is 9 units, -2 to 7, and the vertical distance is -12 units, 9 to -3. The length of the side is found by the Pythagorean Theorem:

A^2 + B^2 = C^2
9^2 + (-12)^2 = 15^2 so C=15. Do the same for the other sides, and the perimeter is the sum of the three sides.