Algebra 1

$5a - 2a = 3a$ and $3b + 7b = 10b$. The two cannot be combined further because $a$ and $b$ are different letters.

$x^2 = 9$, then $2 \times 9 = 18$, and $18 - 5 = 13$. The index applies to the $x$ only.

$3(2x-1) = 6x - 3$ and $-2(x+4) = -2x - 8$. Adding gives $4x - 11$.

Subtract $2x$ from both sides: $3x - 7 = 8$. Add 7: $3x = 15$. Divide by 3: $x = 5$.

Take out the common factor 2 to get $2(x^2 - 4)$, then factorise the difference of two squares.

Subtract $u$ from both sides to get $v - u = at$, then divide both sides by $a$. The whole of $v - u$ goes over the $a$.