You cut the figure into a square and a triangle like you started to.
Then you are given the length of the square as 10. The side length of 5 is only half of the far right side. Double the 5 to get a height of 10.
So right now, you have a square with side lengths of 10. The are of a square is side x side. That means that the area of the square is 10 x 10 = 100 units^2
But we are not finished yet.
Now on to the area of the triangle
Since you know the base of the triangle (right side where you cut the figure into triangle and square) is 10 (because it is equal to the side length of the square) and the height ( dashed line ) is 17, but it is 17 minus the width of the square (10).
So, the triangle's dimensions are a base of 10 and a height of 7.
Area of the triangle is found by Base x Height.
So the area of the triangle is 70 units^2.
Now to find the whole area, you add both of the figures we just found. Add 100 + 70 = 170 units^2!!!
I hope this helped!