The seventh grade mathematics exercise book P66 answers to the second sub-question and the fourth question.

Is triangle ABC the one with an isosceles triangle?

Solution: make the vertical line AD of y axis and the vertical line AE of x axis.

Because angle ADB= angle AEC=90 degrees (vertical meaning)

So ad = 6, AE = 6.

DB=3, CE=3 (known)

In triangle ADB triangle AEC

AD=AE (authentication)

Angle ADB= angle AEC

BD=CE (certification)

So the triangle ADB is equal to the triangle AEC(S.A.S).

So AB=AC (the corresponding sides of congruent triangles are equal).

So triangle ABC is an isosceles triangle