Solve 9x + 15 = −21

The solution of 9x + 15 = −21 is x = -4.

9x + 15 = -21\ \Rightarrow\ x = -4

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The lines y = 9x + 15 and y = -21 meet at x = -4. The solution of the equation is the x-coordinate of the point where the two sides are equal.

How to work it out

1
The goal is to get x alone. Undo the operations in the opposite order: first the addition, then the multiplication.
$9x + 15 = -21$
2
Subtract 15 from both sides:
$9x + 15 - 15 = -21 - 15 \ \Rightarrow\ 9x = -36$
3
Divide both sides by 9:
$\frac{9x}{9} = \frac{-36}{9} \ \Rightarrow\ x = -4$
4
Check: put x = -4 back into the equation.
$9\cdot(-4) + 15 = -36 + 15 = -21 \checkmark$

LaTeX source of the formulas on this page

\[ 9x + 15 = -21\ \Rightarrow\ x = -4 \]
\[ 9x + 15 = -21 \]
\[ 9x + 15 - 15 = -21 - 15 \ \Rightarrow\ 9x = -36 \]
\[ \frac{9x}{9} = \frac{-36}{9} \ \Rightarrow\ x = -4 \]
\[ 9\cdot(-4) + 15 = -36 + 15 = -21 \checkmark \]

Trying values of x in 9x + 15 = −21

x	9x + 15	Equals -21?
-6	-39	no
-5	-30	no
-4	-21	yes
-3	-12	no
-2	-3	no

Questions

What is x in 9x + 15 = −21?

The solution of 9x + 15 = −21 is x = -4.

How do you solve 9x + 15 = −21?

Subtract 15 from both sides to get 9x = -36, then divide both sides by 9 to get x = -4.

How do I check the answer?

Replace x by -4: 9(-4) + 15 = -21, which is -21.

Solve 9x + 15 = −21

How to work it out

Trying values of x in 9x + 15 = −21

Questions

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