let

a = the number of apples

m = the number of melons

the sum of apples and melons is 21

a + m = 21

there are 3 more apples than melons

a = 3 + m

by solving the system of equations

a + m = 21

a = 3 + m

we find

a = 12 apples

m = 9 melons

click here to see the step by step solution of the system of equations

there are 12 apples and 9 melons.