Is this equation linear or nolinear a 2 over b 8?

there's no equals sign so it's not an equation, it's an expression. i assume you mean (a^2)/(b^8) in which case it's a non-linear expression in both a and b.

"linear in x" means the highest power of x in the expression is 1. this means there is no x^2 or x^8 etc.

2x + 4

is linear because the highest power of x is 1

2(x^3) + 4x + 7 is nonlinear because the highest power of x is 3