Simplifying
7 + X + -1(1.25(3.5 + X)) = 0.1
7 + X + -1((3.5 * 1.25 + X * 1.25)) = 0.1
7 + X + -1((4.375 + 1.25X)) = 0.1
7 + X + (4.375 * -1 + 1.25X * -1) = 0.1
7 + X + (-4.375 + -1.25X) = 0.1

Reorder the terms:
7 + -4.375 + X + -1.25X = 0.1

Combine like terms: 7 + -4.375 = 2.625
2.625 + X + -1.25X = 0.1

Combine like terms: X + -1.25X = -0.25X
2.625 + -0.25X = 0.1

Solving
2.625 + -0.25X = 0.1

Solving for variable 'X'.

Move all terms containing X to the left, all other terms to the right.

Add '-2.625' to each side of the equation.
2.625 + -2.625 + -0.25X = 0.1 + -2.625

Combine like terms: 2.625 + -2.625 = 0.000
0.000 + -0.25X = 0.1 + -2.625
-0.25X = 0.1 + -2.625

Combine like terms: 0.1 + -2.625 = -2.525
-0.25X = -2.525

Divide each side by '-0.25'.
X = 10.1

Simplifying
X = 10.1