If 2x + 5y = 21 and x + y = 3, then what is the value of x? Concept Questions System of simultaneous linear <br>equations

System of simultaneous linear equations

1) If 2x + 5y = 21 and x + y = 3, then what is the value of x?

-8
-7
-2
-9

Solution

Answer:

-2

Explanation:

We have a pair of linear equations
2x + 5y = 21 (1)
and x + y = 3 (2)
Which we are going to solve simultaneously
Use the substitution method.
Subtract x from both sides of equation (2):
x + y - x = 3 - x
y = 3 - x
Substitute y = 3 - x into equation (1):
2x + 5(3 - x) = 21
2x + 15 - 5x = 21 - 3x + 15 = 21
Subtract 15 from both sides:
-3x + 15 - 15 = 21 – 15
-3x = 6
Divide both sides by -3
-3x ÷ (-3) = 6 ÷ (-3)
x = -2