Isolate all $x$'s on the same side.

$3.9x-2.1=11.2x+51.7$

$−7.3x−2.1=51.7$

Isolate all constants on the same side.

$−7.3x=53.8$

Divide by the coefficient of $x$.

$x\approx−7.37$

Notice that both fractions in this problem can be changed to exact decimal values.

$x=2.8$

$2\ +\ ?=3$.
How can the fraction simplify to that?

Refer to the hint from part (b).