a: \(=\left(x+1\right)\left(-5x^2+3x+7\right)\)
b: \(=\left(2x+1+x-1\right)\left(2x+1-x+1\right)\)
\(=3x\left(x+2\right)\)
c: \(=\left(3x+15\right)^2-\left(x-7\right)^2\)
\(=\left(3x+15-x+7\right)\left(3x+15+x-7\right)\)
\(=\left(2x+22\right)\left(4x+8\right)\)
\(=8\left(x+11\right)\left(x+2\right)\)