exam Maths Puzzle Numerical Ability

please show me how to do solve by addition I get confused when one equation is neg and the other is pos. like this problem
x+y=2
x-y=4

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exam Other Question

Floor plans for a building have the four corners to a room located at points (2,3), (11,6), (-3,18) and (8,21). Determine whether the side through the

points (2,3) and (11,6) is perpendicular to the side through the points (2,3) and (-3,18). The height h in feet of an object after t seconds is given by the function below
How long will it take the object to hit the ground?

h= -(16 ft/sec^2)t^2 +(60ft/sec)t+34ft