The sum of n terms is given from the first term and the common ratio by

For your given values of a1=-11, r=-4, n=8, the sum of 8 terms is

If the two sides with length 3 and 4 are the two legs, then the missing side, i.e. the hypotenuse, is indeed 5.
But it could also be the case that 4 is the hypotenuse and 3 is one of the legs. In this case, the missing side is the other leg, so we calculate it using

So, a right triangle with legs
and 3 has an hypotenuse of 4.
4x + 8 = -12 is the equation for this statement
When a number and letter are together, it means multiplication.
So, it is read as "23 times f".