First the first one, you can get two vectors in the plane by doing: B - A and C - A:
Then cross these 2 vectors to get a normal vector to the plane:
Use this normal vector setup an equation with A(1,-2,0):
For the second one, it's a similar problem:
Find 3 points in the plane by plugging in any numbers for s and t. Use those 3 points to find 2 vectors in the plane, cross them like the first problem, this gives you a normal vector, then setup a plane equation for the other plane using the point they give you: (-1,2,0).