## Answer :

### Expression 49:

[tex]\[ -d^4 e^{5 f^6} - d^4 e^{5 f} 6 + d^4 e^5 f^6 \][/tex]

To simplify this, first identify the common factor in all the terms:

**Common Factor:**[tex]\( d^4 \)[/tex]

Factoring out [tex]\( d^4 \)[/tex]:

[tex]\[ d^4 \left( -e^{5 f^6} - e^{5 f} 6 + e^5 f^6 \right) \][/tex]

The terms in the parentheses cannot be further simplified since the exponents and bases are different. Thus, the simplified result is:

[tex]\[ -d^4 e^{5 f^6} - d^4 e^5 f^6 + d^4 e^{5 f} 6 \][/tex]

### Expression 50:

[tex]\[ -2 m^2 n^3 - 9 m^2 n^3 + 15 m^2 n^3 \][/tex]

Combine like terms:

[tex]\[ (-2 - 9 + 15) m^2 n^3 \][/tex]

Simplify the coefficients:

[tex]\[ 4 m^2 n^3 \][/tex]

### Expression 51:

[tex]\[ -6 k^2 x^3 + 19 k^2 x^3 - 7 k^2 x^3 \][/tex]

Combine like terms:

[tex]\[ (-6 + 19 - 7) k^2 x^3 \][/tex]

Simplify the coefficients:

[tex]\[ 6 k^2 x^3 \][/tex]

### Expression 52:

[tex]\[ -8 s^7 t^4 + 3 s^7 t^4 - 9 s^7 t^4 \][/tex]

Combine like terms:

[tex]\[ (-8 + 3 - 9) s^7 t^4 \][/tex]

Simplify the coefficients:

[tex]\[ -14 s^7 t^4 \][/tex]

### Final Simplified Results:

1. [tex]\( -d^4 e^{5 f^6} - d^4 e^5 f^6 + d^4 e^{5 f} 6 \)[/tex]

2. [tex]\( 4 m^2 n^3 \)[/tex]

3. [tex]\( 6 k^2 x^3 \)[/tex]

4. [tex]\( -14 s^7 t^4 \)[/tex]