You need to make the grid pattern and the triangular shape separately.
Make the grid using some squares the the Align tools.
Make the triangle by drawing a 3 sided polygon and a circle, and using Pathfinder to subtract the circle from the polygon.
Then using a Clipping Mask to clip the grid to the new triangle.
Or go to Fiverr and pay some dude 5 bucks to do it for you.