#12sqrt(-98)#
#color(white)("XX")=12*sqrt(98)i#
#color(white)("XX")=12*sqrt(7^2*2)i#
#color(white)("XX")=12*7sqrt(2)i#
#color(white)("XX")=84sqrt(2)i#
#4sqrt(-50)#
#color(white)("XX")=4*sqrt(50)i#
#color(white)("XX")=4*sqrt(5^2*2) i#
#color(white)("XX")=4*5sqrt(2)i#
#color(white)("XX")=20sqrt(2)i#
Therefore:
#12sqrt(-98)-4sqrt(-50)#
#color(white)("XX")=84sqrt(2)i-20sqrt(2)i#
#color(white)("XX")=64sqrt(2)i#