We need to find the last angle of the left triangle first:
All interior angles of a triangle = 180, so last angle must be 180-(66+40)=74
Now we can find the angle left to the missing one in the diagram, which can be found given that a straight line is equal to 180*, therefore the angle to the left of the missing angle is 180-(56+74)=50.
The final step is something we learned in step one, that the sum of the interior angles of a triangle is equal to 180*. Therefore, your answer is 180-(50+40)
=90.
Hope this helped :)