The formula for the area of a triangle is where b is the base of the triange and h is the height of the triangle.
The first problem is simply plugging the numbers into the formula.
The easiest way to solve the second one is to split it into two right triangles. You are given the base, and once you split them, you can use the pathagorean theorem to find the height.
The third one gives you the base, but you must use tangent to find the height.
I hope this helps you out some.