The law of sines is given by:
a/sinA = b/sinB = c/sinC
Take into account that in the given problem you need to know what is the measure of angle C, to be able to use the law of sines.
Consider that the sum of the interioiro angles of a triangle is 180°. Then, you have:
m∠C + 98.4° + 24.6° = 180°
m∠C + 123° = 180°
m∠C = 180° - 123°
m∠C = 57°
Next, use the law of sines with sides a and x, angle A and C:
a/sinA = x/sinC solve for x
(a/sinA)(sinC) = x
x = (a/sinA)(sinC) replace the values of known parameters (a = 400)
x = (400/sin98.4°)(sin57°)
x = 339.106
Hence, the length of side x is x = 339.106
