Answer:
in the first one: BMC is equilateral triangle, because BM divides AC into equal sections, and because of that MC is 3.5 so it equals to BC.
Since angle BMC is 60° (because the triangle is equilateral), angle AMC will be 120°.
Since angle A + angle C should be 90°, if amgle C is 60°, then angle A will be 30.
In triangle AMB, angle ABM will be 30° (180°-120°(angle AMB) - 30°(angle BAM)).
As we calulated, angle A is 30° and angle ABM is 30° too, that means in BAM triangle AM=BM.