There are a number of ways. Here are two methods. You will need to draw a picture first.
1. Using a bounding rectangle. You can encase this triangle in a rectangle bounded by lines x=2, x=6, y=1, and y=4. The area of this rectangle is easily calculated as 12 since it is a 4×3.
From the picture, you should see that you can get your triangle by cutting away three right triangles and a smaller rectangle. You can get their areas of the three triangles using A=bh/2. After subtracting their areas away you should get 1/2 for the answer.
2. Pick's theorem. A lattice point is any point with both coordinates being integers. Given a simple lattice polygon, if B is the number of lattice points on the perimeter, and I is the number of lattice points inside the polygon, then the polygon's area is A = B/2 + I - 1.
For your triangle you have B = 3 and I = 0 so the area is 3/2 + 0 - 1 = 1/2.