In order to calculate the length of the line AB, you use the following formula for the distance between points in the coordinate plane:
d = √((x2-x1)²-(y2-y1)²)
That is, it is only necessary to have a pair of points with coordinates (x1,y1) and (x2,y2). In this case you have two points A=(-5,-4)=(x1,y1) and B=(-3,3)=(x2,y2), then, by replacing these values into the formula for the distance you have:
d = √((3-(-3))²-(-5-4)²)
d = √((3+3)²+(-9)²)
d = √(36+81) = √(117) = 10.8166 ≈ 10.82
Hence, the length of the line AB is 10.82