The 3a is squared and the 4x is squared

If i understand correctly, you are solving:

r(3a^{2}) when r(x) = 4x^{2} - 3x + 7

Replace all values of x in r(x) with 3a^{2}

4(3a^{2})^{2} - 3(3a^{2}) + 7

4(9a4) - 9a^{2} + 7

36a^{4} - 9a^{2} + 7