Please see my comment about the equation(s?). To get started, I'll assume you have one equation of
y = (3/4)x - 1
Please note that when you write 3/4x, it means 3 divided by 4x. I chose a form with parentheses, so if you meant the equation as written, my analysis isn't correct.
Perpendicular lines have a slope that is the negative inverse of the first line, In this case, the line (3/4)x - 1 has slope of (3/4). A perpendicular line would have a slope of -(4/3). Enter the data (-3,1) into the new equation (y = -(4/3)x + b) to calculate b, the y-intercept.
I hope this helps,