To solve for \( x \) given the equation \((4x - 5) + (4x + 15) = 90\), we first combine like terms to simplify the equation. This results in \(8x + 10 = 90\). Next, we isolate \( x \) by subtracting 10 from both sides and then dividing by 8.
Step 1: Simplify the Equation
Start with the given equation:
\[
(4x - 5) + (4x + 15) = 90
\]
Combine like terms:
\[
8x + 10 = 90
\]
Step 2: Isolate the Variable
Subtract 10 from both sides to isolate the term with \( x \):
\[
8x = 80
\]
Step 3: Solve for \( x \)
Divide both sides by 8 to solve for \( x \):
\[
x = 10
\]