Solve the system of equations.

To solve the system of equations:

\[
\begin{aligned}
& -4x + 7y = 20 \quad \text{(1)} \\
& y = 3x + 15 \quad \text{(2)}
\end{aligned}
\]

we will substitute equation (2) into equation (1).

From equation (2), we have:

\[
y = 3x + 15
\]

Now, substitute \(y\) in equation (1):

\[
-4x + 7(3x + 15) = 20
\]

Expanding this gives:

\[
-4x + 21x + 105 = 20
\]

Combining like terms:

\[
17x + 105 = 20
\]

Now, isolate \(x\) by subtracting 105 from both sides:

\[
17x = 20 - 105
\]
\[
17x = -85
\]

Dividing both sides by 17:

\[
x = -5
\]

Now, substitute \(x = -5\) back into equation (2) to find \(y\):

\[
y = 3(-5) + 15
\]
\[
y = -15 + 15
\]
\[
y = 0
\]

Thus, the solution of the system of equations is:

\[
x = -5
\]
\[
y = 0
\]