Solve 7x + 15 = 78

The solution of 7x + 15 = 78 is x = 9.

7x + 15 = 78\ \Rightarrow\ x = 9

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The lines y = 7x + 15 and y = 78 meet at x = 9. 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.
$7x + 15 = 78$
2
Subtract 15 from both sides:
$7x + 15 - 15 = 78 - 15 \ \Rightarrow\ 7x = 63$
3
Divide both sides by 7:
$\frac{7x}{7} = \frac{63}{7} \ \Rightarrow\ x = 9$
4
Check: put x = 9 back into the equation.
$7\cdot9 + 15 = 63 + 15 = 78 \checkmark$

LaTeX source of the formulas on this page

\[ 7x + 15 = 78\ \Rightarrow\ x = 9 \]
\[ 7x + 15 = 78 \]
\[ 7x + 15 - 15 = 78 - 15 \ \Rightarrow\ 7x = 63 \]
\[ \frac{7x}{7} = \frac{63}{7} \ \Rightarrow\ x = 9 \]
\[ 7\cdot9 + 15 = 63 + 15 = 78 \checkmark \]

Trying values of x in 7x + 15 = 78

x	7x + 15	Equals 78?
7	64	no
8	71	no
9	78	yes
10	85	no
11	92	no

Questions

What is x in 7x + 15 = 78?

The solution of 7x + 15 = 78 is x = 9.

How do you solve 7x + 15 = 78?

Subtract 15 from both sides to get 7x = 63, then divide both sides by 7 to get x = 9.

How do I check the answer?

Replace x by 9: 7(9) + 15 = 78, which is 78.

Solve 7x + 15 = 78

How to work it out

Trying values of x in 7x + 15 = 78

Questions

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