Hello, Takita!

Great awareness so far of the Distributive Property so far!

\(\displaystyle \L \;21x\,+\,28\,=\,16x\,+\,40\,+\,13\)

First since you have normal, like natural numbers on the right side of the equation, combine them:

\(\displaystyle \L \;21x\,+\,28\,=\,16x\,+\,53\)

Now you can either subtract the 21x or the 16x from both sides of the equation; But the subtraction of 16x causes no negatives for the coefficient x so let's do that:

\(\displaystyle \L \;5x\,+\,28\,=\,53\)

...Now just subtract 28 from both sides:\(\displaystyle \L \;5x\,=\,25\;\;\Rightarrow\;\;x\,=\,5\)