Since a triangle has 180 interior degrees, and 81+57=138, and 180-138=42, you need to find _*2+18=42. (Hint, you can subtract the 18 from the 42, and find _*2=24).
We are given the expression:
U = E * P
The equality symbol “ = “ means that for all cases, the
value of U and E * P must always be equal. This means that whatever happens on
the left side, must equally happen on the right side.
So what this actually means is that, if we double U so we
must also double the expression E * P, that is:
2 U = 2 (E * P)
However since P stays constant in the process, therefore
only E is changed, hence:
2 U = P * 2 E
So we can see that E is also doubled.
hope this helps