F = ma, or F=m(aX+aY)

They've given you m and aX + aY. Calculate the overall FX and FY.

Use the overall FX and FY, subtract out F1 and F2. The resultant components should define F3. Watch your signs and your units and it should be straightforward.

If it's computer graded (as appears) make sure you use three significant figures and a comma: F3x,F3y