Factoring quadratics

I have two problems that I need help with:
1. -28y^2+7t^2
2. 8a^4+27ab^3

Thanks :)

1. -28y^2 + 7t^2

common factor of 7

7(t^2 - 4y^2)

difference of two squares...

7(t + 2y)(t - 2y)

check

2. 8a^4 + 27ab^3

common factor of a

a(8a^3 + 27b^3

sum of two cubes...

a(2a + 3b)(4a^2 - 6ab + 9b^2)

check

@Σ