Can somebody me understand why the measure of angle a is congruent to 30 degrees but isn’t acute? ?

it’s about negation

Angle A is originally equal to 30 degrees and it is originally acute. When you negate it, you flip what the original statement is saying.

So originally angle A is 30 but now it flips to not being 30
Angle A is originally acute, but it flips to not being acute.

note: "negation" is effectively the same as "opposite"

Another note: those two statements are separate it seems. If you say an angle is not 30 degrees, then there is no way to guarantee it is acute or not acute. 

Look at this and see if it