Question

Solve by finding the value of y in the 1st equation.

3x + y = 10
x - y = 2.

Answer 1

Not sure about the phrasing here, there are many ways to find y in the first eqn.
3x + y = 10 (1)
x - y = 2 (2)
solving eqn (1) for y gives
y=10-3x
sub into (2):
x-(10-3x)=2
4x-10=2
4x=12
x=3
back to our expression for y:
y=10-3x=10-3(3)=10-9=1
x=3 y=1

Answer 2

3x + y = 10 ----- (i)
y = 10 - 3x

Substituting this value of y in (ii)
x - y = 2. ----- (ii)
x - (10 - 3x) = 2
x - 10 + 3x = 2
4x = 2 + 10
x = 12/4
x = 3
now substitute this value of x in (i)
3(3) + y = 10
y = 10 - 9
y = 1

so solution set (x,y) = (3,1)