The combined land area of the countries a and b is 186,973 square kilometers. Country a is larger by 373 square kilometers determine the land area of each country

So a+b=186973

a is bigger than b by 373

that means

b+373=a

so we subsitute

a+b=186973

b+373+b=186973

subtract 373 from both sides

b+b=186600

2b=186600

divide both sides by 2

b=93300

subsitute

b+373=a

93300+373=a

93673=a

a=93673 km^2

b=93300 km^2