PLEASE HELP.Consider ABC with the measure of angle B equal to 60 degrees, and side lengths a=4 and c=5. Which option lists an expression that is equivalent to the length of side b?

we know that
the law of cosines establish 
b²=a²+c²-2*a*c*cos B
in this problem
a=4 units
c=5 units
B=60°
so
b²=a²+c²-2*a*c*cos B
b²=4²+5²-2*4*5*cos 60°
b²=16+25-40*(1/2)
b²=16+25-20
b=√[16+25-20]

the answer is
b=√[16+25-20]