The image contains a set of problems illustrating the application of the difference of squares formula: $a^2 - b^2 = (a - b)(a + b)$. Let's solve each expression:

1. $51^2 - 49^2$

Using $a = 51$ and $b = 49$: $51^2 - 49^2 = (51 - 49)(51 + 49) = 2 \cdot 100 = 200$

2. $96^2 - 42^2$

Using $a = 96$ and $b = 42$: $96^2 - 42^2 = (96 - 42)(96 + 42) = 54 \cdot 138$ Calculate: $54 \cdot 138 = 7452$

3. $232^2 - 172^2$

Using $a = 232$ and $b = 172$: $232^2 - 172^2 = (232 - 172)(232 + 172) = 60 \cdot 404$ Calculate: $60 \cdot 404 = 24240$

4. $(7.9)^2 - (2.1)^2$

Using $a = 7.9$ and $b = 2.1$: $(7.9)^2 - (2.1)^2 = (7.9 - 2.1)(7.9 + 2.1) = 5.8 \cdot 10.0$ Calculate: $5.8 \cdot 10.0 = 58.0$

5. $(9.4)^2 - (0.6)^2$

Using $a = 9.4$ and $b = 0.6$: [ (9.4)^2 - (0.6)^2 = (9.4 - 0.6)(9.4 + 0.6) = 8.8 \cdot 10.