We can do this as follows: (Note: In copying the LaTeX from Tekmaker and pasting it here the third equation has been renumbered to (1)!!!. The third equation below should be numbered (3), of course.)
$$Let's first rewrite the two given equations as:
\begin{equation}
y1 = -\frac{3}{2}x+\frac{5}{2}
\end{equation}
\begin{equation}
y2 = -4x+5
\end{equation}$$
$$and the line perpendicular to the first of the above as:
\begin{equation}
y3 = mx+c
\end{equation}
where $m$ is its gradient and $c$ is its intersection with the y-axis.\\\\$$
$$The gradient, $m$, is given by -1/the gradient of the line to which it is perpendicular, so $m=\frac{2}{3}$\\\\
To find the point of intersection of lines (1) and (2) we need to equate them:
$$ -\frac{3}{2}x+\frac{5}{2}=-4x+5$$ and solve for $x$. This is straightforward and results in $x=1$. Plugging this back in either equation (1) or equation (2) we find that the value of y at the intersection point is also $y=1$\\\\
Now we put the known values of $x$ and $y$ at the intersection point, together with our value for $m$ into equation (3):
$$ 1=\frac{2}{3}*1+c$$
$$from which we get $c=\frac{1}{3}$; so, using equation (3), we get the equation of the perpendicular line as:
$$ y3=\frac{2}{3}x+\frac{1}{3}$$$$
Let's plot a graph to see if the result looks sensible:
+33661 
