Using the law of sins:
side a / sin (angle A) = side b / sin(Angle B)
Side a is opposite angle A, so from the picture we know side a = 50, angle A is 58 degrees and angle B is 48 degrees.
replace those values in the formula we get:
50 / sin(58) = b/ sin(48)
so now we can cross multiply the 2 fractions to get:
50 * sin(48) = b*sin(58)
to get b by itself, divide both sides by sin(58) to get:
b = (50*sin(48)) / sin(58)
b = 43.82
You may need to round the answer as needed.